Combine cornstarch, flour, water, cream of tartar, salt, and vegetable oil in a saucepan.

Cook over medium heat, stirring constantly.

Continue stirring until the mixture gathers on the spoon and forms a dough (approximately 6 minutes).

Transfer the dough onto waxed paper.

Let the dough cool slightly until it is safe to handle.

Knead the dough until it is smooth.

Store the play dough in a covered container or plastic bag.

Add food coloring, if desired.