Fix a running toilet in 10 minutes (no plumber needed)

A toilet that runs nonstop is usually an $8 flapper, not a $150 plumber visit. Diagnose in this order:

• The jiggle test: jiggle the handle. If the running stops, the chain is catching — shorten it so there's about an inch of slack.
• The flapper: take the tank lid off and look at the rubber flapper at the bottom. If it's warped, slimy, or doesn't seal flat, replace it. Costs a few dollars, no tools needed — unhook the old one, clip the new one on, reconnect the chain.
• The water level: if water is flowing into the overflow tube (the tall open pipe), the fill valve is set too high. Turn the adjustment screw or clip so the water stops about an inch below the top of the tube.
• The fill valve: if you hear constant hissing and the tank never quite fills, the fill valve is worn. Also a cheap part, also no special tools — shut the wall valve off first.

Safety note: tank water is clean supply water, same as your tap. And always know where your wall shutoff valve is before you start touching things.

Flapper looks fine but it still runs? Do the dye test: put a few drops of food coloring in the tank, don't flush, wait 15 minutes. Color in the bowl means a slow flapper leak you can't see. No color means the fill valve is the culprit.