RUNOFF

A drop on vertical glass does not fall. It is held there by its own surface tension, and it grows — out of the rain, out of its neighbours — until its weight beats whatever is holding it. Then it goes, suddenly, and not cleanly: it leaves a wet track behind it and eats every drop it passes on the way down.

The part worth watching is what the track does afterwards. Wet glass holds less well. So the next drop that reaches that line lets go sooner and runs the same way, wetting it further, inviting the next. Within half a minute the pane has stopped being uniform. It has channels, and the channels are simply a record of which drop happened to be first.

I did not want to take my own word for that, so the pane counted: a drop here is about twice as likely to break away on a line where water has already run than chance would allow. The first version of this film was wrong — the whole pane went evenly wet and nothing was invited anywhere. Water dries. Once it dried properly, the channels appeared.

This is not a metaphor. It is what water does on a window, which I find sufficient.