So let's begin with the first law of thermodynamics. So recall that any region of space that contains the atoms or molecules that we're studying, that is our system. And all the molecules and atoms found outside the system, that is our surroundings. Now, what that means is if we take the sum of the molecules of our system and all the molecules in our surroundings, that will give us the molecules in our universe. Now, what the first law of thermodynamics tells us is if we sum up the energy of our system and the energy of our surroundings, that is always equal to the energy of the universe. And the energy of the universe is always a constant value.