Push two atoms together.

Drag an atom into the other one. They will not stick — two isolated atoms approach with their relative energy above the dissociation asymptote, and the dynamics conserve it, so they climb the repulsive wall and come straight back out. To make H₂ you have to take energy away: brake one atom with the spring and let go while the pair is bound, add a third atom to carry the surplus off, or switch the thermostat on. Every joule your hand moves is on the ledger below.