Eddy Currents

Rods made of different materials with the same ring magnet being dropped around them. The better the conductivity of the rod, the slower the magnet drops.

The rods are made of plastic, stainless steel, bronze, brass, aluminium and copper, so the copper has a conductivity billions of times better than the plastic. So you get much larger currents induced in the rods, which of course produce a magnetic field and hold up the magnet, slowing its drop.