One of the most noticeable behaviors of seismic waves is that they follow strongly curved paths (Figure 8.29). This occurs because the velocity of seismic waves generally increases with depth. In addition, seismic waves travel faster when rock is stiffer or less compressible. These properties of stiffness and compressibility are then used to interpret the composition and temperature of the rock. For instance, when rock is hotter, it becomes less stiff (imagine taking a frozen chocolate bar and then heating it up!), and waves travel more slowly. Waves also travel at different speeds through rocks of different compositions. Thus, the speed hat seismic waves travel can help determine both the kind of rock that is inside Earth and how hot it is.