Hard water has minerals suspended throughout the liquid. This isn't ordinarily a problem, but when you heat up water in a teapot, those minerals can deposit on the metal of the pot. At first, you might not notice the build-up, but over time you'll see a white film on the walls of the teapot. Since these deposits are made from minerals -- just like rocks -- they do not just wash off. Fortunately, there are tricks for removing them that don't require scrubbing.
Pour two cups of vinegar into the teapot.
Heat the vinegar until it boils.
Reduce heat and let simmer for 10 minutes.
Pour the vinegar out of the pot and rinse well with water before using.