The starting point of kinetic theory is an idealised model: identical particles in disordered motion that do not interact with each other. From this minimal assumption comes all the power of the model, which explains pressure as an effect of collisions with the walls and reduces the internal energy to the sole kinetic energy of the molecules. This section introduces the definition, the long history of the idea of the atom, and the decomposition of energy that makes the ideal gas so simple to treat.