Everybody knows how if you take the lenses out of a pair of polaroids, and place them end to end, you can rotate one of them until the light passed through both of them diminishes almost to zero.

Here's an interesting experiment: Next time you have 3 polaroid lenses handy, set up the first two to duplicate the experiment above, rotating one until the two allow no light through at all. Then have a friend insert the 3rd lens in between the other two, so all three are in line, and suddenly you can see through all three lenses! They pass light easily, even though 2 did not! Why is that? Someone told me once there's some obscure quantum physical reason for it, but I don't know. I do know it works because I've tried it.