It takes mere minutes to mix up a batch of squishy stuff that kids and adults alike will find irresistible. You may want to make several batches so everyone has enough to play with. Pour one box of cornstarch into a large foil pan. Add small quantities of water to the powder while mixing. Stop adding water as soon as the powder has dissolved. The mixture should be between a solid and a liquid — runny only when you hold it in your hand. It is not an exact science, so patience is the answer. Mix slowly. Add food coloring for a fun effect. To clean up, just add water — the whole mess will wash away easily.