The word sound has several meanings:

1. Something you hear. A sound is any noise, whether it's music from a piano, your dog barking, thunder rumbling, or your friend laughing. The sound of rain on the roof might help you fall asleep, while the sound of the school bell tells you class is starting. Scientists explain that sounds happen when something vibrates and creates waves in the air that travel to your ears.
2. To seem or appear a certain way when you hear about it. When your teacher describes tomorrow's field trip, it might sound exciting. If someone tells you about a movie and you say, “That sounds boring,” you mean that based on what you've heard, it seems boring to you. Your mom might say a plan sounds good after you explain it.
3. Healthy, strong, or in good condition. A sound building has no structural problems. A sound argument in a debate has solid reasoning with no logical holes. When a doctor says someone is of sound mind, they mean the person thinks clearly and can make good decisions. A sound sleeper rests deeply without waking up constantly. People sometimes say advice is sound when it's reliable and sensible.
4. To measure the depth of water, usually by dropping a weighted line. Sailors sound the water to avoid running their ships into shallow areas or underwater obstacles.