What is sound?
Sound is what is produced when something vibrates in such a way that someone can hear it. The sound is heard when mechanisms inside the ear send electrical impulses to the brain.
This definition enables us to claim that sounds can exist even when there is no-one to hear them and also that objects can vibrate while not necessarily producing sounds.
Sounds have a number of simple properties which can be observed by children and can be understood in terms of the kinds of vibrations made.
Vibrations are to and fro movements and can occur in solids, liquids and gases. If the vibrations are sufficiently rapid (about 20 times per second) but not too fast (about 20,000 times per second) then humans can hear them as sounds. We can see fairly slow vibrations when we "ping" a ruler or an elastic band and we can often feel vibrations when we speak with our fingers lightly touching our vocal chords either side of the neck. Vibrations in slow motion would be similar to the swing of a simple pendulum. The pendulum bob moves first one way and then back again and then repeats this pattern, often with a slightly smaller swing. If you pluck a guitar string you can see it vibrating for a number of seconds afterwards but eventually the amount of movement to and fro becomes too small to be detected by the human ear.
Energy is needed to start something vibrating and to keep it going as in an electric buzzer, a vehicle engine or the human voice. Without an additional source of energy vibrations will gradually become smaller and thus the sound will fade away.