A toilet that keeps running is almost never a mystery once you understand what lives inside the tank. In nearly every case, one small part has stopped doing its one job: either something will not seal, so water leaks out of the tank as fast as it fills, or the fill valve will not shut off, so it keeps pouring in.

Inside every tank-style water heater is a long metal rod, usually running from the top of the tank almost to the bottom. It is called a sacrificial anode, and the word "sacrificial" is the whole point.

Reverse osmosis, usually shortened to RO, is one of the most thorough ways to clean drinking water inside a home. It does not use chemicals or heat. It uses pressure and a very fine barrier to separate clean water from almost everything dissolved in it.

Hard water carries dissolved calcium and magnesium. Those two minerals are the reason a kettle scales up, a shower head clogs, and soap refuses to foam.

A slab leak is a leak in a water line that runs under or through the concrete foundation slab your house sits on. In a slab-on-grade home, the supply pipes for both hot and cold water are often buried in or just beneath that concrete.

Hydrogen sulfide is the gas behind the rotten-egg odor, and your nose is remarkably good at catching it. People can detect it at concentrations well below one part per million, which is why a faint trace at the tap seems so strong.

Older homes were plumbed with air chambers to absorb this shock, short capped stubs of vertical pipe near fixtures that hold a cushion of trapped air. Air compresses; water does not. When that cushion is present, the shock wave pushes into the air pocket instead of the pipe.

Hard water is water carrying dissolved minerals, mainly calcium and magnesium. Those minerals are invisible in the glass, but when the water is heated or left to evaporate, they drop out of solution and cling to whatever surface is handy.