Geyser

A bulbous metal device strapped to your bathroom wall, usually up near the ceiling where your water pipes emerge. Insidiously coiled inside is a heating element that will bring the bathwater within to a near boil on an as-needed basis so you can have a hot bucket bath or in the case of the privileged sections, a shower. For this just-in-time hot water supply chain to function effectively, it requires one essential action on your part: you have to switch on the geyser in advance of your bath.

The switch will either be a monstrous bakelite affair embedded in a large ceramic trip circuit, where flipping it is like throwing a fire alarm lever and a large light will glow red just above the switch to indicate that water is being vapourised.

Or the switch will look surreptitiously like the light switch right next to it and the next time you are done urinating you will hear your dad scream approximately 15 minutes later: "Dai!! Why do you always leave the geyser switch on!!"

Why is this contraption called a Geyser? Well, because each time the water comes to a boil and the whole mechanism shudders and shakes against its restraints bolted to the wall with 1-inch lugnuts, one thinks of Old Faithful or how Iceland gets all its power from Geothermal.