Ask the guest to point a finger at the wall, how high will reach the book, which he holds in his hands, if you place it upright on the floor. When he will do it, put in a fact book on the floor: you will find that its height is almost twice lower than that!
Especially good experience, if requested himself not asked to specify the height, but only on the words tells you where walls need to make a mark. Of course, the experience can be done not only with books but also with lamp, hat and other items that we normally have near the level of our eyes.
The cause of the error lies in the fact that all objects are reduced when we look along them.