The first is goop or oobleck. It's cornstarch and water in equal parts. It's super messy.
Then we tried gak. It's equal parts glue and starch. It's not quite as messy.
I'm thinking about making a slime for Joshua's birthday party. I'm kind of leaning toward gak just because of the mess factor. Though, either one will require a drop cloth of some kind, so who knows. Any opinions for a group of 3 and 4 year olds?