Toss a pebble in a pond -see the ripples? Now drop two pebbles close together. Look at what happens when the two sets of waves combine -you get a new wave! When a crest and a trough meet, they cancel out and the water goes flat. When two crests meet, they produce one, bigger crest. When two troughs collide, they make a single, deeper trough. Believe it or not, you've just found a key to understanding how a hologram works. But what do waves in a pond have to do with those amazing three-dimensional pictures? How do ....