Page "Maxwell's demon" Paragraph 6

In other words, Maxwell imagines one container divided into two parts, A and B.

Both parts are filled with the same gas at equal temperatures and placed next to each other.

Observing the molecules on both sides, an imaginary demon guards a trapdoor between the two parts.

When a faster-than-average molecule from A flies towards the trapdoor, the demon opens it, and the molecule will fly from A to B.

Likewise, when a slower-than-average molecule from B flies towards the trapdoor, the demon will let it pass from B to A.

The average speed of the molecules in B will have increased while in A they will have slowed down on average.

Since average molecular speed corresponds to temperature, the temperature decreases in A and increases in B, contrary to the second law of thermodynamics.