Amazing Optical Illusions

The human eye is a tricky mistress; we trust her at our peril.  She says that two lines are equidistant and we believe her, then the next thing we know, we're waking up in Bangkok with a missing wallet, a headache, and a tattoo we'll never be able to explain to our wife.
Try these amazing optical illusions on yourself.  If that doesn't work try them on someone else.

These two figures seem exactly the same in every way.  And yet the one on the left is entirely different.  Don't ask me why this is.

Stare at these concentric circles for an hour without blinking.  Not easy, is it?

If you drink two martinis very quickly, the object will seem to rotate counter-clockwise.  Drink three, and it will rotate clockwise.  Drink five and it will disappear entirely.  Science has not been able to explain this.

Would you say this shape is a hexagon or an octagon?  Either way, you'd be wrong.

This shape looks a square, doesn't it?  Measure the sides and you'll find they are all exactly the same.  Measure the angles and you'll find each one is exactly 90 degrees.  It looks exactly like a square, and any measurement you can perform on it will show it is a square.  This is because it really is a square, which only goes to show how little we really understand.