Imagine you have a bowl of milk in front of you. Add Cheerios, so you have one clump floating in milk on one edge of the bowl. Then add another clump of cereal - Rice Krispies, this time - on the other edge. Swirl the milk a little with your spoon, and watch the two clumps collide together. Swirl the milk again, and see that the clumps separate... but some Cheerios drift away with the Rice Krispies, while some Rice Crispies stayed behind with the Cheerios lump.
OK, that's the Grenville Event (uh, somewhat simplified... So put on your thinking cap and follow the links below.)
