Every sound is made by something vibrating (moving quickly back and forth). When you pluck a guitar string, it vibrates. When you speak, parts of your throat called vocal cords vibrate. Put your fingers gently on your throat and hum: you can feel the vibrations.

The vibrations pass from particle to particle through a material, so sound can travel through gases (like air), liquids (like water) and solids (like wood or a wall). Sound cannot travel through empty space, because there is nothing to pass the vibrations along.

Pitch is how high or low a sound is. Faster vibrations make a higher pitch. On a stringed instrument, shorter, tighter or thinner strings vibrate faster and make higher notes.

Volume is how loud or soft a sound is. Bigger vibrations make a louder sound. Plucking a string harder makes it louder, not higher.

We hear when sound vibrations travel into our ear and make the thin eardrum vibrate. Messages then travel to the brain. Very loud sounds can damage our hearing, so it is wise to protect our ears.