My cats are compulsive about opening closed doors

[Godhumor]

One has taught the other how to hook her paws under the door and "grasp" the other side to swing open doors on the same side as the hinges, so now they can both get into the closets. Every night I go to bed and the doors to the bathroom, hallway closets and bedroom closets are closed...every morning they're all opened up enough to fit a cat.

I don't even know why they do it; they open the doors and just walk away after glancing inside.