A solid 3mm metal rod can slide (almost) effortlessly through the block. How can that be?
The mechanism for this puzzle is related to the ancient Heron's Horse, where the magician slices a sword through the neck of a horse state without the head falling off. However, whereas Heron's mechanism (see page 109) uses rotors, Oskar's mechanism features meshing gears. The result is a less friction and a smoother cut.