Optical Illusions!

DO THIS: Look at these striped lines. Are they parallel, or do they slant? Grab a ruler and find out the truth.
WHAT'S UP: The lines seem tilted, but they're actually straight and parallel to each other. It's the uneven pattern of the squares that disrupts your brain's perception. If the squares were aligned vertically, the effect would disappear.

DO THIS: Let your eyes roam quickly over the colourful coils in this design. Then watch as they begin to rotate.
WHAT'S UP: Each circle is a set of smaller and smaller rings whose colour patterns do not line up. But your brain wants to keep things simple and orderly, so it tries to match up the same colours by "twisting" the rings in your mind. This makes the circles appear to rotate.

DO THIS: Gaze at the picture. The places where the lines converge into small circles will start to flicker and pulse.
WHAT'S UP: The muscles of your eyes work to focus both eyes on one place. But it's hard for both eyes to see the converging narrow lines in this pattern at the same time. Your brain tries to combine the view seen by each eye, causing some parts of the picture to look as if they're vibrating.

DO THIS: Look rapidly around this picture. It will look as if it's being pulled to the center.
WHAT'S UP: Your eyes move involuntarily many times a second. These motions are so small you can't even feel them. Once your eyes shift, your brain may still see an afterimage of the view you had. When your new view overlaps the afterimage, your brain gets confused and thinks the shapes in the picture are moving.

Here is a cool video on optical illusions.