Have you ever seen pictures of water in motion, where a moment is frozen in time? Now imagine being able to interact with the water while it's still frozen in time. You really need to see the video to understand exactly how cool this is.
The basic idea is to have a steady source of drips and add a strobe light so that each drip is illuminated at the exact same point (or multiple points) in it's descent. You can change either the rate of the strobe or the rate of the dripping water to make it appear like time is moving very slowly, is completely stopped, or even moving backwards!
Man, I wish I had thought of this - it's just so damn cool.