Before you start:The water prism starts as a flat rectangle of clear plastic, heat-folded into a three-dimensional triangular tube. Two small squares are glued on the ends. The prism will be filled with water and sealed. Although I have cluttered the page below with detailed measurments for using commonly available sizes of plastic in the USA with minimal cutting, they are just suggestions. If your country sells plastic based on metric sizes, understand the basic ideas of building the prism and use whatever size is available.

If you can borrow or use on location a "strip heater" it is much easier for heating up where the plastic bends than using a stove. Perhaps a local technical school with a plastics program or look up "plastics" in the phone book will yield information.

What you need:

"Plexiglas," "Lucite" or other clear acrylic window replacement plastic sheet. NOT "Lexan."

IMPORTANT NOTE ABOUT CUTTING THE PLASTIC: Steps 1, 2 and 3 go into great detail how to cut the plastic by measuring, scoring and snapping it for people who want to learn this interesting skill. However, if you would rather not deal with that, just ask the people at the store you got it from to cut it (they will do it quickly with a saw). At my local building center they make the first cut free, then 25 cents for each additional cut. For each prism, you need to end up with:

*1 piece 8" by 10" , plus or minus 1/4"

*2 pieces 4" by 4" , plus or minus 1/4"


8" by 10" (approximately) is a standard off-the-shelf, already-cut size at building centers like Home Depot. 8" by 10" is the first choice for making a single prism, if you can find it. You will need two sheets. One sheet is already the perfect size for the prism. Don't worry about the thickness of the plastic, just get whatever is thinnest and cheapest. In fact, if it's too thick, sawing it will be the only way to cut it (you won't be able to score and snap it). There will be a paper or plastic protective covering which you can peel off now. Be careful what the plastic rubs against so it doesn't get scoffed up.

11" by 14" is another standard size if you can't find 8"x10". You will have to make a few more cuts to get everything sized right, and it will take more physical strength to snap the cuts. You will only need one sheet.

By the way, the same place that sells you the plastic might have a tool for scoring (scratching) so that you can snap the plastic on the score line.

Hot glue gun and glue sticks

There are really two kinds of glue gun: cool melt (which has the thinner glue sticks) or real hot melt (bigger glue sticks). I think the real hot melt glue sticks much better, although it's more likely to burn you.

You could substitute 100% silicone caulk instead of hot glue to hold the prism all together and seal it, but it takes a day to dry (cure), and is harder to repair leaks.

Utility knife or single edge razor

You don't need these if they cut the plastic at the store. To cut the sheet plastic, first score (which means scratch) it, then snap (break it on the straight, scored line. A utility knife or single edge razor is probably the easiest way scoring the plastic. Or for best results buy a special scoring tool from wherever you got the plastic.

Corrugated cardboard

You don't need this if you have access to a strip heater. This is the stuff typical shipping boxes are made from. You will need 3 rectangular pieces, 3" by 8". I find it easiest to cut the top flaps off boxes, then cut them to size. Some of the edges of corrugated cardboard will be wavy, as shown in the illustration. We are going to use it to insulate most of the plastic from heat, allowing full heat to reach the plastic only in strips where we want to bend it.

Ruler and straight edge

The ruler is for measuring, but it can serve as the straight edge as well if it is long enough to span the plastic. We will use the straight edge to guide the razor as we score the plastic.

small amount of sandpaper, masking tape, magic marker, drill with about a 1/4" bit, and a stove.

The stove can be gas or electric. Using a large electric heating element is easier because the heat is more spread out. You can use a gas burner if you keep moving the plastic to spread out the heat. If you can't get a drill, you can heat up a large nail and melt a hole through the plastic. Do it outside, though, because it smells nasty.

Any kind of gloves, mittens, pot-holder mittens, or just rags

Any of these will protect your hands while you fold the hot plastic into a triangle.

Or a strip heater

Using a strip heater to heat plastic is unquestionably easier than using a stove and cardboard masks. They cost about $100. A teacher who works with plastics might take a interest and let you use one.

Step 1

Note: If you had them cut the plastic at the store, skip to step 4.

Measure, score the plastic sheet.

If you have sheets that are about 8" by 10", one piece is already the right size. You only need to cut the two 4" square ends from the other piece. First measure 4" and from the 8" end, and mark (red in the illustration) on two sides.

Use a straight edge to connect the lines. We will use it as a guide to score (scratch) the line. Put down an old magazine or something to protect your work table. Use the utility knife or single edge razor to scratch the plastic. Push down hard on the blade and go back over it several times. In addition to holding the straight edge down with your free hand, it's a good idea to to tape it, too. There is nothing worse than having the straight edge slip when you are half way through.

If you have a 11" by 14" piece of plastic, measure the 4" from the 11" side and mark it on each side (red in the illustration at left). Scratch a score line between the marks , as explained above.

Step 2

Snap on the score line.

Snapping means breaking the plastic on the scratch line to cut it. It's an interesting process, and it is the same way brick, glass and stone is often cut: scratch, then break on that weak part.

Snap the plastic on a sharp edge of a table. Line up the score line (red in the illustration) with the edge of the table (brown in the illustration). IMPORTANT! Make sure the scratched side of the plastic is facing up! Hold the part of the plastic on the table down with one hand. Grab the edge of the other part of the plastic that's hanging over the edge of the table and use all your body weight to push down really, really hard.

If it worked, go to step 3. If it doesn't snap the first time, make sure the scratch is aligned with the table edge each time. You might have to get some help to push the plastic down with enough force to snap it (make sure you both push down at the same time). If it still doesn't snap, try the alternate method explained in the next paragraph.

If you have a 11" by 14" piece of plastic it will be even harder to break because there is more of it. Try breaking it on the table edge. Consider getting some help. If you still can't get it, try this alternate method: You will need a wooden board (perhaps a cutting board) and an uncarpeted floor. You might be able to substitute the bottom of a drawer for the board. Stick part of the plastic under the board so the scratch lines up with the edge of the board. IMPORTANT! Since we will be pulling up this time on the plastic, the score line must face down! Grab the edge of the plastic with both hands and kneel on the board. Pull up with all your might. I have been able to snap some large pieces of plastic this way that would not break otherwise .

If you still can't get the plastic to break on a score line, it can be sawn on a table saw or radial arm saw, though it stinks when you cut it.

Step 3

A little more cutting

If you started with an 8" by 10" piece, you should now have an 8" by 4" piece broken off if all went well in steps one and two. From this piece we need two 4" by 4" squares. Measure in 4", mark, score and snap the rectangle in half. Go to step 4.

If you started with an 11" by 14" piece, we will get the 4" squares from the 11" by 4" piece that you have broken off. Measure 4", then 8" from one end. Score and snap. You will have a piece of plastic 3" by 4" left over. See the "Exploring Water Prisms" page for some things to do with the scraps.

Now we'll get that 8" by 10" piece from the large piece of plastic left. Measure 8" from the 10" side, scratch the score line, and snap. This size is quite hard to break. Score thoroughly and consider using the method explained at the end of step 2.


Step 4

Lay out where to bend the plastic

Industrially, we would use a "strip heater" to heat just the strip of plastic where we wanted it to bend. We will get the same result by masking all of the plastic (blue)-- except the strips we want to heat-- with 3 pieces of cardboard (brown) . The air spaces in the corrugated cardboard are pretty good insulators.Then we selectively heat the exposed strips over the stove.

Start by cutting corrugated cardboard into three rectangles, each 3" by 8". Bend as little as possible while cutting.

Put two of the pieces of cardboard on top of the 8" by 10" piece of plastic (blue) and align the corners, as shown.

Peel off a strip of masking tape (light blue) that is about 20" long. Don't try to use several shorter pieces instead of the single long piece. Center the tape on a cardboard piece and stick it on. Then, wrap both ends around to the other side of the plastic,pull tight, and stick them there. The cardboard should now be taped to the plastic, its corners aligned with the corners of the plastic. The reason we use a single, long piece of tape is that the cardboard side gets hot. The tape does not stick when hot. But on the plastic side, it is cooler. It stays stuck, holding on the cardboard from the cool side. If there were a break in the tape on the hot side, it would peel off.

Take the last piece of cardboard and center it in between the other two so the spaces are equal. Attach it with tape as you did for the other two. When turned over so they are on the bottom and held over a heat source, these spaces allow the exposed plastic to heat.

The next piece of tape will form a handle to hold onto while heating. Peel off one more very long piece of tape. Center it on the middle piece of cardboard, over the piece of tape already on it. But this time, instead of wrapping it tightly around the other side, loop it very loosely on the other side. This creates a handle. By holding onto this tape handle on top, It's a good idea to wrap another piece of tape as a handle for strength.

Step 5

Heat and bend the plastic

The fragrance you create as you heat the cardboard, tape and plastic sheet will not likely endear you to the homeowner. Use the exhaust fan and open the windows if possible.

When we heat the plastic, it will be several inches away from the flame or heat source. The temperature will not get hot enough to set the cardboard on fire, but clear your sink out so you can throw it in if you are the one-in-a-million who does. When you hold the sheet over the heat, you should not see smoke coming from it. When it does smoke--don't fret--just lift it farther from the heat and/or turn the heat down. I give the sheet as much heat as I can short of making smoke.

I think it is easiest to heat both of the bending strips at the same time. Keep the sheet moving to distribute the heat evenly to all parts of the strips. This is especially important if you are heating over a gas burner, which has a concentrated column of heat. After several minutes of heating, you will sense a slight bending at the two exposed strips. Don't stop heating yet. Let it keep heating another minute or two to be sure it will bend easily.

When it is melted enough that you don't have to use lots of strength to bend it, fold the the outside thirds touch each other. Use gloves or rags so you don't get burned. If you start bending and it seems stiff, stop bending and heat it more. If you force the bend, it will break. Even it does break or crack, you can still seal it with hot glue (next step).

Fold and hold the corners together while the plastic cools. If they pull apart a little as they cool, that's OK. Remove the cardboard and tape.

Step 6

Glue the ends on and seal

Gluing and sealing the prism is mostly a common-sense operation if you remember these tips:

*Hot glue is forgiving in some ways but not in others. Even if you have a leak or an end comes off the prism, you can re-glue it. However, hot glue! Hotter than boiling water, in fact. Be careful!

*Taking a few seconds to scuff (make lots of tiny scratches on) the smooth plastic where the glue will stick helps the glue stick better. This is particularly true of the plastic ends. Use any scrap of sandpaper. Or just take a bit of gritty soil and rub it where the glue will go. A common complaint of plastic windows is that they scratch much easier than glass--but that's an advantage when you're trying to get hot glue to stick.

*The key to neat hot glue jobs is to put on a thin layer and let it cool before putting on more. If you try to do it all at once, you'll be dripping glue all over the place. If you do get a big drip, remember that you can heat it up with the tip of the glue gun and re-form it.

With the above tips in mind, temporarily tape the edges of the triangle together if there is a gap. Glue the edges together, except where the tape is. Overlap the glue over each edge at least 1/4". When the glue has cooled, remove the tape and fill in the gaps with the glue. When that has cooled, go back over looking for leaks.

Although it is optional, it is a good idea to sand the ends where the glue will go. Whenever I have seen a prism come apart, it has always been an end coming off--the glue staying stuck to the triangle but not to the end. I have never seen an end that was sanded come off. Put the triangle on an end and trace it with a magic marker. Scuff this part up.

Glue the ends on. I suggest that the glue strip that holds on the end be 1/2" wide for lots of holding power, but do it in several passes. Check all around the seams for possible leaks.

Step 7

Make a hole, fill with water, and plug

I make the hole to fill the prism with water on one end, although you could put it anywhere. A drill with a bit 1/8" or over will do the job. A 1/4" hole fills quickly. If you don't have a drill, heat up a nail on the stove and--holding it with pliers--push it through the plastic.

Plug the hole with tape, preferably electrical or duct tape. The surface must be dry for the tape to stick. Once you are sure there are no leaks, you can plug the hole with hot glue if you want to (it's harder for curious fingers to open).

To fix leaks, figure out where it's leaking, empty the prism, and let it dry out before gluing.


For explanations, activities and cool links related to water prisms and spectrums click here or on the picture.

Back to the water prism introduction page.

Back to the science toymaker home page.

I'd like to know how this project goes for you. I'm happy to answer questions about it. Feedback from you is an important way for me to know what works and what needs clarification.
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