Tuning fork vibrations disturbing water. A tuning fork is a piece of metal with two tines (prongs) that vibrate at a set frequency when struck. The vibrations of the tines agitate surrounding air molecules, which then agitate more distant air molecules and so on. This forms a sound wave. However, the vibrating air molecules cannot be seen. Instead, the fork can be placed in water. The disturbance of the water illustrates the agitation of the air molecules.