Convection currents are movement of fluid as a result of differential heating or convection. In the case of the Earth, convection currents refer to the motion of molten rock in the mantle as radioactive decay heats up magma, causing it to rise and driving the global-scale flow of magma. The density of a liquid reduces as it is heated. If the liquid is heated far from the surface, such as at or near the bottom of the container, its reduced density is displaced by colder, denser liquid, and the lighter liquid rises to the top where it meets the cooler liquid. The end consequence is that the heat transfer creates a circulation effect up until the boiling point, or the temperature at which all of the liquid is the same. The lesson to be learned from convective heating is to avoid heating liquids traditionally from the surface because doing so will not result in convection currents. The electromagnetic energy from microwaves typically heats from the top down, penetrating just a few millimeters. The movement (agitation) of the water container and the turbulence caused by the boiling surface water turning into steam are what enable heat transmission. It is typical to see superheating with explosive results due to the localized energy absorption by the surface portion of the water, especially if a powdered substance is introduced to the microwaved water. When the powder is mixed with hot water, nucleation sites are created, which might result in a quick ebullition of boiling water.