conduction

Conduction is how heat travels through something solid by direct contact, passed along from particle to particle without the material itself moving. Stir hot soup with a metal spoon and before long the handle you are holding turns warm too, because heat has crept up the spoon by conduction.

At the microscopic level, faster-moving particles in the hot region jostle their slower neighbours, handing on some of their energy, which passes step by step through the material; in metals, freely roaming electrons carry heat as well, which is why metals conduct so well. The rate of heat flow through a slab obeys Fourier's law, Q/t = k A ΔT / L, where A is the cross-sectional area, L the thickness, ΔT the temperature difference across it, and k the material's thermal conductivity.

Materials that conduct heat poorly (air, wool, foam, wood) are called insulators, and they work by slowing conduction down; a woolly jumper keeps you warm mostly by trapping still air, one of the poorest conductors. Conduction is the dominant mode of heat transfer through solids, whereas moving fluids carry heat mainly by convection and empty space only by radiation.