You can perform a number of optical illusions using mirrors.
This illusion works because our eyes and brains tell us that the reflection of the leg in front of the mirror is actually the leg behind the mirror.
Suppose you have your right leg in front of the mirror. In the mirror, the students see the inside of your right leg facing them, in the exact orientation they’d expect to see your left leg.
The light bouncing off the mirror acts as though it comes from a leg behind the mirror, in the exact location where we would expect to see a left leg.
When you lift the leg in front of the mirror, the reflected leg also lifts. The illusion is that both feet have lifted from the ground!
This demonstration is so good you will think you are flying!