Hearing and sound
The sense of hearing is what makes it possible to speak and listen to others. It also brings us the sensation of lots of sounds, some of which are pleasant like the musical notes of instruments or birds and some which are harsh like the noise produced by machinery.
But to hear anything, sound has to be created first. Sound is created when something vibrates. For example, when we bang on the table, the table is set into vibrations, so small, that we cannot see it. These vibrations are then transferred to the air that is surrounding it. When our ears catch these vibrations travelling in air we associate what we hear with the banging of the table. Similarly, when we talk, vibrations that are caused in our throat (Voice box) are passed on to our ears through air. Some medium is necessary for sound to travel from one place to another. Air is one medium. Other mediums are liquids and solids.
You can hear sound travelling through water when you are swimming. Or simply close one ear, put the other in a bucket of water and scratch the bucket. If you repeat the same exercise by lifting your ear out of water, you will notice a change between the sounds you heard, one travelling through the water medium and the other travelling through the air medium.
Tightly shut your ears. Grind your teeth. Can you hear sound? Now open your ears and again grind your teeth. You can again hear the sound. You will also notice that the two sounds are different. In the first case, the grinding of teeth set in vibrations, which were transmitted to your ears through the jaw bones in your face. Because you had closed your ears, you blocked the vibrations that were travelling through air. In the second case, you heard both. Vibrations that reach you through air and vibrations that reach you through the bones. Thus the same sound is heard differently when transmitted through air (gas) or water (liquid) or a solid medium.