Today I was taking the train to a nearby city, where we passed through some rainy region. As the raindrops fell on the window, one could see their trajectories almost followed a parabola depending on their initial conditions. That is something expected, due to gravity and the forces acted on them by the running wind. However as you see them moving, they leave behind a water tail - something like a stream-, and that tail is the preferred way the rest drops will follow (they move up to it, and then they follow it). Why is this happening? Also if there are no more drops to follow that tail, it will suddenly break apart to stationary drops. Why is this happening? I don't think it's due to statistics, because if it was that, you wouldn't see the whole line to be divided simultaneously. It's more like, due to tension forces on the water, it's an unstable "structure"- under some perturbations it falls apart all together. I hope I made the questions clear. Looking forward to your answers.