Haven’t tried this yet, but it sounds like fun for kids and child-like grownups!

1-1/4 t Borax
1/2 C Water
1/4 C White Glue
1/4 C Water
Food Coloring (optional)
Ziploc bag

Stir the Borax into the cup of water until dissolved.
Mix the glue and the quarter cup of water.
Put the two solutions in a Ziploc bag with some food coloring, close the bag, and knead.

Keep this in the refrigerator when you’re not playing with it. I don’t know how long it lasts, but if it gets moldy, toss it and make another batch.