Sound is just a vibration, like a jiggle or a shake, that moves through things. This "things" is called a medium, and it can be air, water, or even something solid. You can't have sound in a vacuum because there's nothing for the vibration to travel through.

Why do we hear it? The vibrations from a sound hit our eardrum and make it shake. Our brain figures out what those shakes mean and that's how we hear things.

How does sound travel? Sound moves in waves. The little particles in the air get pushed together and then spread apart, which makes the wave move. Sound moves fastest through solids because the particles are packed in tight, and slowest in gases like air. People always think loud sounds go faster, but they don't. Speed is speed, loudness is loudness.

What is pitch? Pitch is how high or low a sound is. A high pitch is like a whistle, and a low pitch is like a deep voice. This depends on how fast the waves are. The faster the waves, the higher the pitch. We call this frequency.

What is loudness? Loudness is how strong the sound is. It's about how big the waves are. A big wave means a loud sound, and a small wave means a quiet one. This is called amplitude. It is not pitch.

• The speed of sound in air is about 343 meters per second.

• Sound waves are longitudinal waves. This means the particles move back and forth in the same direction the wave is going. This is different from light waves, which are transverse . A transverse wave is a type of wave where the vibration of the particles is at a right angle (90 degrees) to the direction the wave is moving..

• The normal human ear can hear sounds between 20 Hz and 20,000 Hz.