Smelly chairs

Smelly chairs is a riddle. It can be played with groups of any size. You need at least two players (trainers) who know the correct solution.

Equipment
You need three (or more) chairs.

Procedure
One trainer claims to have a very good smelling sense. He can smell, if somebody sat on a chair.

To prove that, he puts three empty chairs in the center of the room and he leaves the room. The group selects one player, that sits on only one of the chairs. Then, the trainer is called in again. He smells on all three chairs, and then tells the group the correct chair. How did he do that?

Solution

 * You can find the solution at smelly chairs/solution.

Sesselriechen