It is 1834, and you have just heard of this marvelous new phenomenon called eddy currents. Some fellow named Lenz discovered them, and you’re curious if you can find out something special about them yourself.
When you drop your magnet through a copper tube, it slows down. The magnet will also slide down the cookie sheet slowly.
Magnetic fields are the result of electric currents. Changing a magnetic field next to a non-magnetic metal will induce an electric field in the metal, which subsequently generates a magnetic field with an opposite orientation with respect to your magnet.
The magnetic field induced in the copper pipe attracts the falling magnet, creating resistance. This resistance is what slowed down your magnet. As your magnet slows down, it stops generating as much current, which reduces the resistance acting on the magnet’s movement. You will love this experiment!