I recently bought a book called "Eye puzzles" and man, was i amazed.

Well, an ambigram is a picture, word, anything you want it to be which has 2 or more meanings in the picture.

This site demonstrates.
http://ambigram.matic.com/

An amazing picture by master Gustave Verbeek. Turn your head upside down and be amazed =D
http://members.lycos.nl/AmazingArt/i...k_birdboat.gif

Lots of ambigram words.
http://img287.imageshack.us/img287/5...mbigram2lx.jpg