| Visitors are
invited to move their hands rapidly around a darkened box. Photocells
in the box trigger a strobe lamp every time a hand passes over them.
When the light is off, the box is dim and it is difficult to see
the hand at all. Because the hand can only be seen clearly in the
place where it triggers the strobe, it seems to be standing still
in that position, even as the user moves it around the box.