One of the delights of having small children is that they make a great audience for simple tricks and old gags that older kids have either already seen or are too world-experienced to appreciate. Here's one such trick. All it takes is a carton of orange juice.
Get a carton of orange juice, one of those half-gallon waxed cardboard containers with a screw-on top. Make sure it is about three-quarters empty. Unscrew the cap and place it lightly on its hole. Hold the carton with your right hand, and give it a sharp squeeze. The cap will jump off the carton, sailing a good three feet if you do it right.
Now you are ready to amaze your small child. You tell him you can open the orange juice without even touching the cap. Ask the kid to count to three, and use your left hand to snap your fingers at just the right moment, pointing at the cap. Your right hand executes the cap-lifting squeeze. If you play up the role of the left hand enough, the child will be watching it, and won't notice the right hand at all. Suddenly you're Superman!
The trick won't mystify them for long, but even after they see through it, it's a fun trick to let them try themselves. Aren't kids great?