I made Gak for Lili and her friends to play with for playgroup today. It's turned out to be a great thing, as she's been playing with it for a very long time by herself this afternoon, and it is so much less messy than playdoh!
It's really easy to make, too:
Solution A: 4 oz (1/2 cup) Elmer's White glue (generic would work too)
1/2 cup warm water
Solution B : 1 1/3 cup hot water
4 tsp Borax (powdered laundry booster)
Mix both solutions separately until well mixed, and/or dissolved.
Pour Solution A into Solution B and gak will immediately form at the bottom. Agitate it a little with your fingers, and pour off some of the water off the top. Pull the gak bundle out of the water and just let it flow through your hands back and forth until the extra water comes off. Store in an airtight container for who knows how long. Beware that it can get stuck to clothing, and I don't know (yet) if it washes out!