Sidewalk Chalk.

I open the cardboard box and examine each stick of light coloured chalk. They were no different than any other chalk until you used them. Everything you drew, came to life.

I turn over the box which was covered in nothing but rules.

1. All animals must have round teeth.
2. Stick men are flimsy. Pack them out.
3. To get rid of your creations, use cold water and wash them away.
4.You are responsible for all creations.

They weren't the most enforcable of rules but they were still rules everyone went buy if they used these chalks.