Add a few drops of food coloring to the water, and stir into cornstarch. When the cornstarch is dissolved, drop the whole batch onto a table top for children to play with. Gunk is almost liquid which makes an interesting change from other doughs and clays. This is the cleanest of the messy activities because when the gunk dries, it can be swept away. Any that is leftover can be stored in a clean jar and allowed to dry out. It can be reconstituted by adding water.