1Prepare an ice cube tray.

Prepare a plastic or silicone ice cube tray. Apply a thin layer of petroleum jelly inside each section. This will make it easy to release the chalk from the tray.

2Prepare the paint.

Prepare different colors of poster paint or tempera paint. Make sure they're the washable type so the chalk won't stain the sidewalk!

3Mix some plaster of paris.

The typical ratio for mixing plaster of paris is 2 parts powder to 1 part water. Use this ratio or follow the instructions on the package of your plaster of paris powder

To make one small batch, mix ½ cup plaster of paris powder and ¼ cup water. Follow the mixing instructions on your brand of plaster of paris powder or use this basic mixing guide.

4Add a bit of paint.

Once your plaster of paris mixture is free of lumps, add a small dollop of poster paint or tempera paint.

5Mix well.

Mix gently until the mixture is evenly colored. Add more paint if you want a more intense color.

6Pour the mixture.

Pour the mixture into the sections of your ice cube tray. For a three-colored chalk, fill each section a third of the way; for a two-colored chalk, fill it halfway; and for a solid-colored chalk, fill it all the way.

Tap the tray gently against the table to let the mixture level out.

7Create shallow grooves.

When the mixture is starting to solidify but is still soft, gently carve out shallow grooves across the surface of each block with a popsicle stick or with the end of a spoon. The grooves will help bind the next layer of plaster of paris strongly with the current layer.

8Mix and pour the second layer.

Mix another batch of colored plaster of paris (repeat Steps 3 to 5) if you are making a two-colored or three-colored chalk.

For a two-colored chalk, pour the mixture all the way to the top of each tray section. For a three-colored chalk, pour the mixture two-thirds of the way.

9Make grooves.

When making a three-colored chalk, you will need to carve out some shallow grooves on the surface of the solidifying second layer just like in Step 7. The grooves will help bind the second layer with the third layer.

10Mix and pour the third layer.

Mix another colored batch of plaster of paris. Pour this over the second layer to complete a three-colored chalk. Allow this layer to set completely.

Plaster of Paris usually takes about twenty minutes or so to set. You can leave it in longer just to make sure. You will know when it has set when the chalk is rock-hard and cool to the touch.

11Twist the tray.

Once the chalk has set completely, loosen them from the tray by slightly twisting the tray sideways.

12Remove the chalk from the tray.

Carefully turn the tray upside-down and catch the chalk cubes with your hand or lay them directly on the table.

13Wipe off the grease.

Wipe off the layer of petroleum jelly covering each chalk cube.

14Use the chalk.

Now try out your rainbow-colored chalk. You can draw thick multi-colored lines with just one stroke!