Here's one which puzzles me. We say "turn on the light", meaning cause electricity to flow through a light bulb. But why "turn on"? It's usually a switch with a lever rather than a rheostat.

So I'm assuming that the origin of the phrase will be related to a spigot on a barrel (well, I would, wouldn't I, given my interest in real ale? ). But I can't find any definite origin for the term. Anyone able to help?