Recipe: 1/4 lb of cornstarch, 1 oz of shampoo, warm water, food coloring
Put a few drops of food coloring into a bowl, add the shampoo. Notice how the shampoo flows - that is what scientists call viscosity. Add the cornstarch and mix with spatula. Add warm water a tablespoon at a time and mix after each one. The water make the starch expand forming a network that holds the water and cornstarch together in a slimy mix. Gradually your mixture will turn into a thick paste.
If you pick it up and knead it in your hand it gets gloopy but if you thump it it gets hard.
Why? Starch molecules are bigger than water molecules so if the mixture can move around it stays liquid - but sudden pressure jams the molecules together so the mixture can't flow because you squeeze out the water molecules.
Keep it in a sealed container if you want to use it later.