We hear by means of sound waves, these waves are movements of air which travel from whatever makes the sound to our ears. However, if we are swimming these sound waves are carried by the water.
Sound cannot travel in outer space because there is nothing there to carry the sound waves. An astronaut on a space walk needs a radio intercom to speak with his fellow crewmembers.
The human ear is divided into three parts, the outer, middle and the inner ear. What we usually call the ear is technically only the outmost part. The outer ear consists of the familiar flap of flesh, together with the tube the leads from this to the ear drum.
Beyond the eardrum inside the head, is the middle ear. This is a continuation of the ear tube, which contains three small bones. The first of these is connected to the eardrum and all small bones are connected to each other.
Deep inside the head is the inner ear. The last of the three small bones connects with a coiled part of the inner ear called the cochlea.
How the ear works.
When sound waves reach our ears, they are collected in the out ear, which acts as a funnel. The sound waves cause the eardrum, which is tightly stretched like a drum skin to vibrate.
The eardrum passes on these vibrations to a series of small bones of the middle ear. The last of these passes the vibration to the cochlea, in the middle ear.
It also contains a thin membrane, which lies along the coils of the cochlea. When the cilia and the membrane vibrate, they cause nerves to send messages to the brain. When we say that we hear a sound, this is what we mean.
We hear a difference between high pitched and low pitched sounds because these affect different parts of the cochlea membrane. High pitched sounds cause vibrations at the base of the cochlea. Lower pitched, or deeper, sounds cause vibrations farther along the cochlea.
Besides the parts already described the inner ear also contain semicircular canals filled with a watery liquid. These are not for hear but are used for pout sense of balance. The Eustachian tube connects the inner ear with the throat. If it becomes blocked, air pressure builds up in the inner ear. This is why when sometimes you have a bad cold, which can block the tube, may affect our hearing