Give an example of a change in internal energy through heat transfer.

A pot of water over the fire.
The water in the pot is initially cold, and its internal energy is much less than boiling water. Gradually, the fire heats up the air, and the air, in turn, heats the pot and water. This is heat transfer. Molecules in a liquid begin to move faster, and its internal energy increases.

Hand on cold iron.
Its temperature at the point of contact will decrease, which means that the internal energy will also decrease, while the temperature of iron will increase. This means that the same thing will happen with the internal energy.

Melting metal in a furnace.
When melting, the metal ceases to be solid, that is, its molecules break a structured bond with each other, and this requires a lot of energy, which the metal receives through heat transfer. The transition to another state clearly shows the change in internal energy.