I have done that with clients before. It is a simple mind over matter way of thinking.
Have them sit down, put 20 pounds more on a machine than they are used to doing, tell them it is the same as last time they were there and watch them crank out the same amount of reps. Then you show them what you did and they can't believe it. I always used that as a lesson about how the mind is stronger than the body. Great post!