Limescale is the hard white or grey crust that builds up inside kettles, taps, pipes and water tanks when the water is “hard” — water that carries a lot of calcium. Boil the same kettle for months and you may see a rough white layer on the bottom and around the spout. In much of Britain this is an everyday nuisance; in much of Taiwan the water is softer, so students often meet it first as kettle crust rather than as a plumbing problem. Cleaners and plumbers talk about it all the time. The “lime” here is not the green fruit. It is the calcium material you also meet in limestone. Vinegar or other weak acids can sometimes loosen it.