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Science Experiment: Making Chocolate Legos (and many other things too!)

Key Concepts: Melting Point, Mold Casting, Manufacturing

Today, we’re going to use a food-safe mold to make chocolate replicas of legos, or other hard plastic toys.

Time: 1.5 hours

Cost: ~$40, depending on how many materials you already have. I have included links to some the more obscure products on Amazon, such as the mold material I use and a hot glue gun.


  • 1 package of food safe mold material, I’m using ComposiMold
  • Chocolate Chips or a broken Chocolate Bar (enough to fill the mold, amount will vary depending on how big the toy you cast is.)
  • 1 hot glue gun.
  • 1 flat-bottomed, heat-safe bowl or tray, to cast your mold in.
  • 1 microwaveable bowl or measuring cup to melt the material in.
  • 2 spoons, to stir the mold and the chocolate with.
  • 1 knife, to help pry the mold out of its tray.
  • 1 chopstick or toothpick to brush bubbles out of chocolate during casting (optional.)
  • 1+ legos or other hard plastic toys. Avoid soft plastic as it might melt into the mold. The toys shouldn’t have too many moving parts, unless you don’t mind them getting filled with plastic. Super small thin parts should be avoided, as the chocolate will probably break when you take it out. The toy should not be electronic either. I’m using legos and a plastic snake. The snake might be too long and thin for this to work, but we’ll see!

1) Clean and dry the toys you want to cast. Carefully use some small drops of hot glue to secure them to the bottom of the flat and heat-safe bowl or tray. Make sure that the part with the detail you care about is face up, and the flattest parts are on the bottom.

2) Melt the casting material in the microwave. Set it for 1 minute at a time, stirring after each minute until all the material has melted.


3) Pour the material over the plastic toy, and continue pouring until the toy is covered.

4) Gently use the spoon to brush away any large bubbles forming near the toy, ensuring that no bubbles harden directly next to the toy. Sometimes the toys with air trapped on the bottom will release bubbles due to the warm mold heating the trapped air, causing it to expand and escape.

5) Place your tray in the freezer, let chill for 30-60 minutes (until the mold feels cool and rubbery to the touch.

6) Work the mold free from the tray. It helps to use a knife to pry it loose before working it with your hands. It might be difficult, but it’s OK to mangle it a little, as long as the area with the toy doesn’t get damaged.

7) Pop the toys out of the mold, it’s also ok to work the toys out with a dull knife if they get stuck. Just take care not to pierce the mold, or create any marks that will distort your casting. In the case of the snake, the mold seeped under it, so i had to cut out the bottom layer to free the snake. That’s OK, because the top detail is what we care about.


7) Using the microwave, melt some chocolate chips or a broken chocolate bar into a cup (preferably with a pour spout, such as a pyrex measuring cup) The chocolate will hold its form even after melted, so be careful to stir at 1-minute intervals to test its consistency. Eventually, it should form a thick but smooth fluid.

8) Pour the chocolate into the mold, and use the spoon to help scrape it out if it’s thick.. If there are places for air pockets to form (such as the little bumps in Legos), use a toothpick or chopstick to force the chocolate into the air pocket. This will dislodge the bubble, and cause it to rise to the surface, or at least get stuck in the middle of the chocolate where it won’t be seen. In the photo below (the one that shows the underside of the mold, you can see where I’m poking out the bubbles with a chopstick.

Here, I use the chopstick to dislodge the air bubbles in the lego’s bumps.

9) Spread the chocolate evenly around your mold, and scrape away any extra from the sides to get a cleaner shape. It’s not entirely necessary to remove the spillover chocolate, but it will make it easier to remove the shapes in the end. In mine, I didn’t scrape away the spillover, though I wished I had by the end.

Here, I decided to make my legos more detailed by creating holes in the bottom so my pieces can interlock with each other. I placed the Legos face down over the filled molds, pressing them into the warm chocolate slightly. I also left one with a smooth back.

10) Place the molds back into the freezer, to help the chocolate set faster. When the chocolate is cold and hard to the touch (30+ minutes, depending on your freezer temperatures), you can push the chocolate out of the mold, clean the edges, and eat your chocolate Legos!


I had to use a knife to separate the plastic legos from the chocolate, and I had a bit of work to do with removing the spillover chocolate from the edges. It all worked out in the end, though!. Here, you can see how the chocolate Legos link together using the holes I impressed onto the back.


The snake didn’t work out as well; there were too many thin parts, and too much stress on the chocolate for it to come out in one piece. But even mistakes are tasty!

After you’re finished, you can rinse out the mold with lukewarm water, and re-use it to cast again. If you want to make something different, save it so that you can melt it down again. ComposiMold can be re-melted and used up to 30 times, so definitely don’t throw it away! You can also peel the extra material off of the spoon and bowl you used to melt it for later use.

The Science Behind it All, and Real World Applications:

Most objects you’d find in a house are either cast (what we just did) or sculpted. When you sculpt something, you start with a shapeless piece of material, then carve, hammer, melt, and bend it until it’s the shape you want. A blown-glass lamp, a hand-carved wooden toy, a clay vase, and some pieces of jewelry are made this way. Many other things are cast. Most plastics, lots of cooking glassware (anything with a seam), and many metal objects all started out as a molten goop that was poured into a mold.

The defining principle behind casting materials is melting point differences. Everything has a temperature in which it changes phases, from a solid, to a liquid, to a gas. Challenge your child to think of water: what are the freezing (liquid to solid) and boiling (liquid to gas) points of water? At room temperature, water is a liquid, but if we add heat, we can turn it into a gas. If we take away heat, it can turn into a solid. While for water, this occurs at 32 F and 212F, other things boil and freeze at different temperatures.

Our ComposiMold, our plastic toys, our casting trays, and our chocolate are all solids at room temperature. However, they all turn to liquids at different temperatures. If they all melted at the same temperature, the second we put our glass tray in the microwave, it would become a puddle along with our ComposiMold! When we poured the mold over the legos, the legos would melt into it! And when we poured the chocolate into the ComposiMold, the mold would loose it’s form and turn into a chocolatey, plasticy blob!

Challenge your child to think which items have the highest and lowest melting points, and ask them to put them in order.

For your reference:

A pyrex glass tray melts at 1,510 F

Legos melt at 176 F (for normal bricks) or 267 F (for transparent bricks)

ComposiMold melts at 130 F

Chocolate melts at 86 F

This is why we can pour melted ComposiMold over legos and into a tray without melting them, and we can pour melted chocolate into the ComposiMold without melting it.

Melting point and boiling point differences are very important in chemistry and industry. As every substance has a specific melting point, a starting point for identifying identify mystery chemicals is to see which point they melt at. If somebody gave me a mystery white crystal from the kitchen, I could tell if it’s sugar or salt by seeing if it melted at 366 F (sugar) or 1,474 F (salt).

Differences in boiling point can help us separate the components of a solution: it might seem like a solution of rubbing alcohol and water can’t ever get separated, but as it happens, rubbing alcohol boils at 180 F, a whole 32 degrees below water! We can purify the water by heating it to something like 190 F or 200F, which will make the alcohol boil off before the water does! We can also use a chemistry apparatus to collect the pure alcohol vapor (by forcing it to condense), in a process called distillation. Distillation is a very important process in chemistry; we distill water to purify it, as well as distilling liquors, kerosene, petroleum, and even essential oils.

And finally, just like we used a mold to cast our chocolate legos, real legos are made in much the same way! Lego has industrial-scale molds for all their bricks, which they fill with plastic instead of chocolate. Many other things are cast exactly the same way, with the mold material changed to something with the appropriate flexibility and melting point for the material that it’s being filled with.

For example, to cast a metal object, we can’t use something with a melting point like ComposiMold. Gold melts at 1,948 F which would obliterate the mold, and even the pyrex glass! To cast something like gold, we first carve a wax model of the object we want (let’s say, a ring). Then, we pour plaster over the wax ring to make the mold. After that, we heat the plaster to a temperature where the wax melts, but the plaster doesn’t (plaster begins to deform at 2,200 F). So, once the wax ring melts out, we are left with a perfect impression of it in the plaster mold. Since plaster melts at around 250 F above the gold, it’s safe to fill it with molten gold and let it cool into the ring we want. Then, we can break the plaster to free the ring.

After discussing this with your child, be sure to continue to reference the principles in your daily interactions. Ask them to figure out if your shampoo was sculpted or cast. When cooking, ask them which material might melt first: your spatula or the frying pan? When treating a cut with rubbing alcohol, challenge them to think of exactly how the chemists got it to be a mixture of 91% alcohol. By referencing scientific principles in daily conversation, your child will improve their critical thinking and start to see the world like a scientist does.