A kind of wave is a type of energy that travels through a medium, such as air or water. TIDAL waves are large ocean waves caused by the gravitational pull of the moon and sun. Seismic waves are created by earthquakes and travel through the Earth's crust. SINE waves are smooth, repeating curves that are used to represent periodic functions in mathematics. CRIME waves are a series of related criminal activities that occur in a particular area. Alpha waves are electrical signals produced by the brain during relaxed states. SONIC waves are sound waves that travel through air or other mediums. RADIO waves are electromagnetic waves used for communication. HEAT waves are periods of unusually hot weather. Micro waves are short RADIO waves used for heating food. Marcel waves are a type of wave created by the movement of sand on a beach.