They do it with mirrors

Abracadabra! And the rabbit in the box has completely disappeared – or has it? Mirrors have been widely used to cast illusions for centuries, particularly by magicians. The Mirror Box trick is just one such illusion. By using a mirror slotted diagonally along the inside of the box, the magician creates the impression that the box is empty, its contents having mysteriously vanished. When in reality Flopsy the bunny rabbit is safely tucked in the hidden half of the box, the audience are looking at the reflection of the bottom of the box