Limescale
As the water evaporates, the insoluble calcium carbonate and magnesium carbonate become limescale. If you are living in a hard water region, limescale is a normal phenomenon. If pour over all water immediately, it will easily form Limescale.
Remove Limescale/Spots Quickly: Filling a small amount of WHITE VINEGAR inside and let it stay in the kettle for 30-60 minutes, then empty the kettle.