The answer to 'sink constantly running' is that the most common causes are a faulty flapper valve, a stuck float, or a leaky faucet. This can lead to water continuously running into the sink and wasting a significant amount of water.
If the sink is constantly running, first check the flapper valve in the toilet tank. This is the rubber seal that lifts up when you flush, allowing water to flow into the bowl. Over time, the flapper can become worn or cracked, preventing it from sealing properly and causing a constant trickle. You can try adjusting or replacing the flapper yourself, but if the issue persists, you may need to call a professional plumber.