Almost every share-house I've lived in has had some nasty looking solids collecting inside. I always regret looking inside the kettle because a cup of tea never tastes quite as good after you've seen this:
What is that stuff? What's the physical process that causes it to collect and why is it usually concentrated around the heating element? Is it related to calcium in tap water, or is there any trick to prevent it appearing in your kettle? Is it dangerous to ingest and should be removed, or is it harmless?