Fluid mechanics is the study of the behavior of fluids (liquids and gases). Fluid statics includes the study of pressure, density, Pascal's law and Archimedes' principle. Fluid dynamics includes the study of streamlines (see streamlining), Bernoulli's law, and the propagation of waves. Engineers use fluid mechanics in the design if bridges, dams, and ships. Physicists use it in studying the structure of the atomic nucleus, and astronomer use it to explain the spiral structure of some galaxies.