A sound wave is a vibration that travels through air, water, or other materials, carrying sound from one place to another. When you pluck a guitar string, it vibrates back and forth incredibly fast, pushing and pulling the air molecules around it. These pushes and pulls spread outward in all directions, like ripples moving across a pond, until they reach your ear and you hear the sound.

Sound waves are invisible, but they're always moving through the air around you. When you speak, your vocal cords vibrate and create sound waves. When you clap your hands, the sudden smack sends sound waves racing outward at about 767 miles per hour (the speed of sound). These waves bounce off walls, which is why you hear echoes in a large, empty room.

Different sound waves create different sounds. Fast vibrations make high-pitched sounds with tightly packed waves, while slow vibrations make low, rumbling sounds with waves spread farther apart. Loud sounds come from powerful vibrations that create strong waves, while quiet sounds come from gentle vibrations that create weak waves.

Sound waves can't travel through empty space because they need molecules to push against. That's why outer space is completely silent: there's no air for sound waves to move through, no matter how loud an explosion might be.