The flapper is the most common cause and the cheapest fix. It's the rubber (or rubberized) seal at the bottom of the tank that lifts when you flush and should drop back down to seal the tank from the bowl. Over time it warps, cracks, or collects mineral deposits from hard water and stops sealing completely, letting water trickle continuously into the bowl. A flapper swap is a quick, low-cost repair.
If a new flapper doesn't fix it, look at the fill valve — the taller assembly that refills the tank after a flush. A worn fill valve can run intermittently or continuously, and a fill valve failure often comes with a distinctive hissing sound. Float misadjustment is a related, simpler cause: if the float is set too high, water keeps refilling past the shutoff point and constantly spills into the overflow tube, which reads exactly like a running toilet even though every part is technically working.
Not every running toilet is loud about it. A silent leak — where the flapper isn't sealing but water is draining so slowly you can't hear it — still runs up your water use. The classic test: put a few drops of food coloring or a dye tablet in the tank, wait about 15-20 minutes without flushing, and check the bowl. Any color in the bowl means water is leaking past the flapper. Over weeks, even a slow, silent leak can waste thousands of gallons — a toilet that runs continuously can lose water in the hundreds of gallons per day, so it's worth fixing promptly even when it doesn't sound urgent.