Supplies needed: cornstarch, water, sugar, and plastic bag with a large circle drawn on it.
In a large pot, mix 1 cup of cornstarch with 4 cups of water and 1/3 cup sugar. Stir over low/medium heat until the mixture thickens into goop. I then added it to two bowls before adding a few drops of blue and green food coloring.
Tuck and daddy mixed the goop with the food coloring. We then added the goop to the bag and squished away!
Note: In the classroom, I always taped the seal of the bag with clear packing tape so little fingers couldn't open the bag spilling the contents.