Well, before we open the door, what can we say about the energy in our system? So how much energy is found on this side and how much energy is found on this side? Well, notice that we have six molecules on this side, and each one of these molecules carries a certain amount of energy. So if the molecules are moving, they have a certain amount of kinetic energy and so forth. So each one of these molecules has a certain amount of energy. And the total amount of energy on the left side of this container is equal to the sum of the individual energies of these molecules.