This is the best Canadian Brain Teaser I have ever seen.
It is a "How is this done?" type of teaser!!

A person is given thirteen 3 inch nails.
One nail is hammered into a block of wood and sits perfectly vertical with about 2.75 inches remaining out of the block.
The person is asked to mount the remaining 12 nails atop the vertical nail.

Question : How is this done?

Hint : Magnetism is NOT the answer.

See the answer