All you need is cornstarch and water. Make as much as you want by combining two parts of cornstarch to one part of water. If you want to tint it with food colouring, you totally can.
My kids aren't really into messy things that get them sticky and gooey, but this was a smash hit. Yeah!
What is kind of neat about this concoction is that you can break off a piece of it, crumble it, roll it into a ball, but if you just set it on your hand and stop manipulating it, it starts to 'melt.' Cool!