This recipe was given to me by a classmate in college. She made it for her girls, and now I make it for mine. It's very easy and turns out great every time. The secret is kneading it until it's smooth after it cools.
1 cup water
1 T vegetable oil
1 cup flour
1 packet of sugar-free Kool-Ade (any flavor)
2 T cream of tartar OR alum
¼ cup salt
Combine the dry ingredients in a bowl.
Combine the water and oil in a medium saucepan over medium heat.
Slowly add the dry ingredients, stirring constantly.
Reduce the heat to low and keep stirring until the mixture starts to boil and pull away from the sides of the pan.
Remove the playdough from the pan and place on a piece of wax paper to cool to room temperature.
When it’s cool, knead the dough to make it softer and smoother.
Store in an airtight container.
You can make some substitutions for the Kool-Ade. If you mix in 3-4 tablespoons of pumpkin pie spice and a little orange food coloring you have a great autumn-scented playdough. Leave out the food coloring, add a bit more ginger, and you have gingerbread playdough. If your children fancy chocolate playdough, add about 1/2 cup of cocoa powder in with the dry ingredients and decrease the flour by about half.
I've found that this dough keeps a very long time if you store it in a ziploc bag and squeeze the air out each time you put it away. My girls can play with playdough for hours on end. It's a great rainy-day treat!