Herman's Grid Illusion

In these grids, you can see small grey dots bouncing around in the intersections of the gridlines, except at the grid intersection which you are actually looking at. The dots appear to move because as you move your eyes, the one you try to look at disappears, whereas the one you were looking at reappears.

The reason the dots disappear and reappear is that when you look directly at something, you point your most sensitive vision in that direction, so that you gather information with finer resolution. Everything in your peripheral vision is summed more coarsely, so that at the grid intersections, the surrounding colours differ to those along the grids.