How come the wheels of a moving car appear to rotate backward sometimes?

How come the wheels of a moving car appear to rotate backward sometimes?
Wheels appear to rotate backwards in movies because of what is called, 'strobe effect'.
When filming, the camera actually takes a series of pictures.
An example would be:
Imagine you have a spoked wheel rotating at 30 revolutions per second. If the camera is filming at a rate of 30 frames per second (taking 30 individual pict...ures each second)then the wheel would be in the same position every time the picture was taken. When viewing the film it would appear that the wheel was not moving.
Now, slow the wheel down a little bit and itwould be slightly behind the position it wasin when the previous frame was shot. Now the wheelwould appear as if it is rotating backwards because you are not viewing the continuous motion of the wheel, but rather the positions of the wheel every 1/30th of a second.
You can see the same effect when looking at a fan turning in a room with Fluorescent lighting. The bulbs flicker at a certain cycle thus causing the illusion.