Goop is a solid when you pick it up but quickly turns to liquid and oozes through your fingers.
The boys really liked this activity and quickly learned how to make the ooze last longer by picking up larger handfuls of goop.
To make your own goop you put 1 cup to 2 cups of corn starch in a container and add 1.5 cups of water. I just kept adding water until I got the consistency that I wanted. Cheap and easy fun!