Re: why do astronauts touch their head masks when they have to talk in space?

Sound is vibration. When you speak, you make the air around you shake back and forth, hundreds of times per second. The air near you pushes on air farther away, causing it to shake, and so the vibration moves out, away from you. Solids, liquids, and gases all carry sound in the same way (although at different speeds.)

But you can't have vibration unless there is something to shake. Vacuum is nothing, and so sound can't go through it. An astronaut floating in space who speaks in his space suit makes the air in the suit vibrate, which makes the suit itself vibrate, but the sound can't go any farther, since the suit has nothing to push on.

But if the astronaut touches his helmet to his friend's helmet, then when he speaks, he makes his helmet vibrate, which makes his friend's helmet vibrate, which makes the air inside the helmet vibrate, which his friend can hear.

Touching helmets works, but usually astronauts want to talk without touching. They use radios in their suits to do this. Radio waves can pass through vacuum, and so astronauts use them to communicate with each other, with their spacecraft, and with people on the ground.