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, which can lead to significant damage during an earthquake. S Waves can only travel through solids, meaning they do not pass through liquids or gases. This characteristic helps scientists understand the Earth's internal structure, as the presence of S Waves indicates solid materials, while their absence in certain areas suggests liquid layers, such as the outer core.