Ambigrams!
An ambigram is an image that looks the same upside down as it does right side up. For example, spinning a picture 180o. Look at the following samples of ambigrams. A lot of these came from Dan Brown's thriller, Angels and Demons.

Ambigram of 'air' Ambigram of 'water' Ambigram of 'fire' Ambigram of 'earth'
Ambigram of 'angles and demons'

Ambigram of 'go with the flow'

Ambigram of 'good book'

Ambigram of 'library'

Ambigram of 'sleep'

Ambigram of 'mark'

Fibonacci
Last Supper
phi
Puzzles
Ambigrams