Why does my faucet drip and how do I fix it?

A dripping faucet is almost always caused by a worn or damaged internal component that no longer creates a watertight seal. In a traditional compression faucet, the rubber washer wears out. In ball, cartridge, or ceramic disc faucets, the cartridge, O-rings, or ceramic disc itself deteriorates over time. Mineral buildup can also prevent parts from seating properly. To fix it, first shut off the water supply valves under the sink. Remove the faucet handle by unscrewing the decorative cap and handle screw. Depending on your faucet type, replace the rubber washer, O-rings, or the entire cartridge. Take the old parts to a hardware store to find exact replacements. Reassemble everything, turn the water back on, and test. The whole job usually takes under an hour.