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Heat is a form of energy that manifests itself in the motion of molecules and atoms, as well as subatomic particles. Heat energy can be transferred by conduction, convection, or radiation. In conduction heat spreads through a substance when faster atoms and molecules collide with neighboring slower ones, transferring some of their kinetic energy to them. This is how the handle of a teaspoon sticking out of a cup of hot tea eventually gets hot, though it is not in direct contact with the hot liquid. When a fluid is heated, portions of the fluid near the source of the heat tend to become less dense and expand outward, causing currents in the fluid. When these less dense regions rise, cooler portions flow in to take their place, which are then themselves subject to heating. This current flow is called convection. Many ocean currents are convection currents caused by the uneven heating of the ocean waters by the Sun. Radiation transmits heat in the form of electromagnetic waves, especially infrared waves, which have a lower frequency than visible light but a higher frequency than microwaves. Atoms and molecules in a substance struck by such radiation readily absorb the energy from these waves, thereby increasing their own kinetic energy and thus the temperature of the substance.