An S wave, or secondary wave, is a type of seismic wave that travels through the Earth during an earthquake. Unlike P waves, which are primary waves that move in a compressional manner, S waves move in a side-to-side motion. This movement causes the ground to shake, making S waves responsible for much of the damage during seismic events. S waves can only travel through solid materials, which means they do not pass through liquids or gases. This characteristic helps scientists, such as seismologists, understand the Earth's internal structure and the composition of its layers, including the mantle and core.