1 cup flour
1/2 cup salt
1 tablespoon alum
1 hot cup water, with a few drops of food coloring (makes mixing simpler)
1 tablespoon vegetable or olive oil
For reference: Alum is in the spice aisle. In the As.
Mix everything together, if it's soupy then microwave it for 30 seconds at a time. When you start getting your fingers into it it might still feel a bit thin but as you kneed it with flour it's dry up a bit.
Play away, then put it in a ziplock in the fridge when you're done and take it out the next day, or whenever, ours is keeping pretty well. And yes it seems cold coming out of the fridge but it doesn't take much hand contact to warm it up again.
Josie, age 2.5, wasn't great at making snakes and balls and other things we tried to teach her but she LOVED taking a knife to the stuff, all week she's been hanging out with a butter knife just chopping up the playdough, making "vegetable soup" or ham or cake or whatever she's making, it's cute. And since the homemade stuff costs like thirty cents or whatever to make, I figure she could just play until she's sick of it or it dries out and then we'll make more.
It's a lot like the stuff you buy except not as crumbly, not as artificial-colored looking, and you get a huge hunk of it from just this little recipe. Definitely easy and makes buying the stuff seem lame. Mom and I used to make it for neighborhood kids, like for Christmas presents, they were always very impressed.