Sound is a form of energy that spreads out evenly in all directions from a vibrating source, through a medium such as air or water, and at a constant speed. When a spoon strikes a glass, the glass begins to vibrate back and forth very quickly. The vibrating glass causes the air molecules next to it to be pushed together and then pulled apart, creating alternating regions of higher and lower air pressure called compressions and rarefactions. The compressions and rarefactions continue to form and travel in longitudinal waves away from the glass until the glass stops vibrating. (It's important to note that the air molecules themselves don't actually travel away from the source -- the energy does. The molecules simply transfer this energy to the molecules they bump into.) When the waves collide with our eardrums, we hear sound.

The frequency of a sound wave is the number of areas of compression that pass by per unit of time. The greater the number of compressions, the higher the frequency. The pitch of a sound -- how high or low it seems -- depends on the frequency of the sound wave. High-pitched sounds have high frequencies; low-pitched sounds have low frequencies.

Consider the glass and spoon once more. An empty glass, when struck, produces a sound with a certain pitch. But when you add water to the glass and strike it again, the sound produced is lower in pitch. Why is this so? By adding water, you introduce more matter that needs to vibrate in order to create sound. This slows the vibrations so that there will be fewer compressions per second produced in the surrounding air.

By changing certain variables in the example above, such as the amount or type of liquid you add to the glass or the size or shape of the glass itself, you can affect frequency and thus pitch. Generally speaking, the larger and heavier an object is, the more material there is to vibrate. The more material there is to vibrate, the slower the vibrations, and the lower the pitch of the sound produced.