This is really cool and super quick/easy. Combine the following in a little bowl:
Then add some water to make a nice smooth paste.
Paint away on a nice thick sheet of cardboard (if you don't have enough paint brushes use cotton buds - they work really well).
Microwave the design on high for 10 - 30 seconds until the paint puffs and it's all nice and dry. We did ours for 30 seconds.