In every solid, conduction begins with lattice vibrations

1. Heat one end of a solid and the particles there gain kinetic energy, so they vibrate more strongly about their fixed positions.
2. Those particles are packed tightly against their neighbours, so a stronger vibration means they collide harder with the particles next to them.
3. Each collision passes on some energy, so the neighbours begin to vibrate more strongly in turn, and the effect travels along the solid from the hot end towards the cool end.
4. The energy moves but the particles do not, because each one stays around its own fixed position throughout, and that is what makes this conduction rather than convection.
5. Passing energy neighbour by neighbour is a slow way of moving it, which is why non-metallic solids such as glass and brick conduct poorly even though this mechanism works perfectly well in them.

In metals, free electrons carry the energy far faster

1. A metal has the vibration mechanism as well, but it also has a sea of free electrons moving randomly through the whole lattice.
2. At the hot end these electrons gain kinetic energy and move faster.
3. Being free, they travel right across the metal rather than staying put, carrying that energy with them.
4. When they reach the cooler part they collide with the atoms there and hand over energy, making those atoms vibrate more.
5. This route is far quicker than the vibration route, because the electrons are light, fast, and cross the whole distance themselves instead of relaying energy from one neighbour to the next.
6. Both mechanisms run at once in a metal, but the free-electron mechanism dominates, and that is the whole reason metals are the good thermal conductors.
7. The same free electrons also carry electric current, which is why good electrical conductors are usually good thermal conductors as well.

Gases and most liquids conduct badly because the mechanisms barely work

1. In a gas the particles are far apart, so a fast particle travels a long way before it meets another one and collisions are much less frequent.
2. There is also no fixed arrangement in a gas, so there are no lattice vibrations to pass energy along, and energy can only be handed on collision by rare collision.
3. In a liquid the particles are close together, so collisions are frequent, but they are not held in a rigid arrangement and so cannot pass on vibrations efficiently.
4. Neither gases nor ordinary liquids have free electrons, so the fast mechanism is missing altogether, and the exceptions are the liquid metals such as mercury, which conduct well for exactly that reason.
5. Gases are therefore the worst conductors of all, which is why trapping still air in a cavity wall, a duvet or a jumper is such an effective way of keeping energy where you want it.
6. The air has to be kept still for this to work, because moving air carries energy by convection and undoes the benefit.