What is sound

Sound is vibration in the air. Sometimes you can see the vibrations; sometimes you can not. If you pluck a taut rubber band, you can see the band twang. If you clap your hands, you see nothing. Vibration is there all the same. But it is not only the band or your hand move, you hear sound because air moves too. As the source of sound moves, it sets air molecules moving too & so vibration is transmitted through air to ears.
Can sound travel in vaccum?
No. Sound can travel through solid, liquid & air. In fact, it can travel faster in solid & liquid than air, because their molecules are closely packed. But there is complete silence in vaccum as there is nothing to vibrate.
What are sound waves?
Sound waves are not like waves in the sea, which go up & down. Sound waves moves by alternately streching & sqeezing. When a sound is made, air molecule near source are sqeezed together. They in turns jostle up against the molecules next to them & then are pulled back into place by molecules behind.
How do we hear sound?
You hear because your ear pick up tiny vibrations in the air made by the sound. Inside ear is a taut, thin window of skin called airdrum, which vibrates with the air & rattles three tiny bones called ossicles. The last of them called stirrup, rattles faster than first to amplify sound. Stirrup shakes another thin skin, sending waves through a fluid filled tube called cochlea, deep inside your head. Ripples in cochlea trigger sensetive nerve receptors to send signals to your brain.