Like light, sound travels through the air in waves, but unlike light, sound is not made of lots of tiny particles. When something makes a sound, like you clapping your hands, it’s because when you clapped your hands that shook the air molecules around your hands and made them vibrate (that means they shake quickly back and forth). This vibration, in turn, shook the air molecules a little further away from your hands, and they shook the air molecules next to them, and so on, until the air molecules inside your ear were vibrating too (and inside the ears of the people sitting near you too).
When the air molecules inside your ear begin to shake, they wobble tiny hairs inside your ear that are connected to nerves under your skin. If your ears are working, these nerves then send messages to your brain to tell you that you heard a noise.
Because sound has to move molecules in order to travel, it’s impossible for sound to move through space, where there are very few molecules. Space is a very quiet place. But sound doesn’t have to move through air – it can just as easily move through water, or through metal wires. In fact, sound moves faster through water than it does through air.
But whether in air or in water, sound moves much more slowly than light does. While light travels at 186,000 miles per second, sound only goes 0.2 miles per second (343 meters per second, or about 770 miles per hour). A fast airplane can go faster than the speed of sound. Because of this, you often hear things long after you saw them. For instance, you have to wait several seconds to hear the thunder after you see the lightning in a storm, even though they are the same thing.