Which one and only one of the nine arrows' tips points exactly into the centre of the respective circle?

http://www.puzzles.com/PuzzlePlayground/PointTheCenter/PointTheCenter.ht...

There are three coins placed as shown in the illustration. Can you slide the middle coin to the right until distance AB equals distance CD?

http://www.puzzles.com/PuzzlePlayground/CoinDistances/CoinDistances.htm

A nice little collection of interactive optical illusions.
http://www.poissonrouge.com/bigfish/en/opticalillusions.html