My French pen pal is here! Ma correspondent Eulalie! When we speak to each other (mostly in English... they start about 5 years before we do), it is sometimes a little difficult to understand each other. However, the fact that we can hear each other is up to physics. When we speak, we trigger vibrations that travel through the air in longitudinal waves, meaning there are a series of pulses that form compressions and rarafractions in the particles of the air. The sound energy that is carried by the pulses reaches my ears or hers, depending on who is speaking.