You mix some corn flour in a bowl with some water. Borax+water works too!
water with salt with flour with glue
my suggestion:
Goop:
8 oz. white craft glue
1 cup warm water
1 cup cold water
1 1/2 teaspoons of Borax powder
4 drops of food coloring
step 1. mix warm water and borax powder together untill it dissoves
step 2. in a seperat container mix glue and normal water along with the food coloring
step 3. mix the borax water and the foor coloring water.
step 4. stir for about a minute and you will see results!