The vibrations from an object cause the air in its surroundings to produce sound waves, which then transmit the sound outward in different directions. When the waves reach your ears, they are transformed into messages in the form of sound. Your brain picks up the messages then processes them so you are able to identify the kind of sound you heard.

Sound travels at different speeds, depending on the kind of matter making the sound and the medium through which the sound travels.

Matter is made up of molecules, or very tiny particles. The molecules of solids are compressed and strongly attached to each other so sound waves pass easily through solid materials. On the other hand, the molecules of liquids are less attached and move slower. This causes sound waves to travel more slowly in liquid materials.

The molecules of gases are much farther from each other, making it more difficult for sound waves to travel through them.

Sound travels so much more slowly than light. Light moves at about 300 000 000 meters per second while sound travels at only 340 meters per second. This explains why you may hear the sound of an object long after you have seen it. Notice how, during a thunderstorm, you see flashes of lightning before you hear the sound that they make.

There are no particles of matter in outer space. Thus, sound waves are not able to travel in it. Inside a spacecraft, astronauts can talk to each other because there is air inside the spacecraft. However, when they are out in space, astronauts use special instruments that allow them to talk to and hear each other.