a magnet

two nails

 

 1. Approach a nail to the magnet, then approach the nail to the other nail.

The first nail attracts the second.

2. Leave the nail stuck to the magnet, keeping it stuck to the magnet only on one end.

The first nail attracts the second and they remain united.

3. Remove the magnet

The nails remain stuck one to each other 

The magnetic force is transmitted to the nail and becomes much stronger than the one in the magnet.