A wave is a method of transferring energy from one place to another without transferring matter. Mechanical waves are those that require a medium for their transfer and include water waves, sound waves and waves in stretched strings. A disturbance at A causes a disturbance of a particle, that drags its neighbour's particles along with it until the disturbance reaches B. If the disturbance at the source continues, the wave is maintained, and if it is simple harmonic, then a plot of the displacement of the particles at a single point in time is a sine curve. This is the basic nature of a mechanical wave that is considered when looking at mechanical waves.