A fascinating Web site, River Runner, charts the path of a raindrop from any place within the continental USA to the sea. You click on a map with your mouse cursor, or enter the name of a town in a search box, and the Web site calculates the path from your chosen starting point, along the various rivers and valleys that lead from there to the sea, on either the East or the West coasts or in the Gulf of Mexico. The map then gives you a visual impression of the path, running at a speed that you control, showing you the route of the various waterways and landmarks along the journey.
Here's an example of how the map looks (although, of course, this screenshot doesn't show it in motion). Click the image for a larger view.
It's a fascinating exploration of geography, and how landscape channels water (and is itself modified by water). In particular, one sees how some areas are more affected by drought than others, because they offer fewer pathways for water to reach them and/or flow across them.
Recommended viewing for all ages, but particularly for schoolchildren.