Have you ever wanted to paint right on sidewalk, and then wash the evidence away? This is a fun, messy way to make paint that will do just that!

[edit] Steps

Bring your materials outside, to a cement surface where you can make a bit of a mess.
Take the paintbrush and use the plastic handle, rubbing it on the side of the chalk in order to shave off fine dust. Angle the dust so it falls into the cup.
Slowly pour water into the cup of chalk powder. Use the handle of the paintbrush to gradually mix the water into the powder, stirring it until smooth.
Dip your brush into the paint. Draw pictures unto the sidewalk.
When you're done with your artistic masterpiece, simply wash the paint off with water.

[edit] Tips

Try mixing different colors of chalk to make interesting combinations.
Instead of shaving off dust, you could grind the chalk up with a stone. This, however, will produce more coursely ground powder, and therefore lumpy paint.

If it's a very hot day, you may want to quickly wash off the paint before the sun bakes it into the cement. It won't stain or mark the sidewalk, but it will be harder to wash off. You may have to scrub it off if it's hard enough!