The way a toilet works is a mystery to most people. But like many mysteries, a closer examination may reveal that the toilet isn’t really very mysterious at all. In fact, this device is ingeniously simple in both design and function.

Mostly likely, you’ll be dealing with a gravity-fed toilet—and luckily, nearly all of your problems can be traced to three parts: the fill valve, the flapper, and the tank lever. Familiarize yourself with this trio of hardworking parts and you’ll be able to fix most toilet failures without calling a plumber.