Sound needs a medium

When I speak, I push the air next to my mouth. That air pushes the air next to it, and so on, until it reaches your ear. So sound needs something to travel through.

Through solids: put your ear on a desk while a friend gently knocks on the other end. You hear it clearly, through the wood.

Through liquids: tap two spoons together under water. You can still hear them.

The material sound travels through is its medium: a solid, a liquid or a gas. A space with no matter at all is a vacuum. (Put a finger over the nozzle of a pichkari and try to pull the plunger: you are trying to make a vacuum, and it's hard.)

The bell jar experiment. An electric bell rings inside a glass jar. A pump slowly sucks the air out:

• as the air goes, the ringing gets fainter;
• with almost no air left, you hear almost nothing, even though you can see the bell still ringing;
• let the air back in, and the sound returns.

So sound cannot travel through a vacuum.

In space. Outer space is nearly a vacuum. Two astronauts on a spacewalk, side by side, cannot hear each other directly. They talk through radios fitted in their spacesuits.