The Master Tao realised that many of his students restrict themselves and never reach their potential, because of the limitations they put in there minds.
One day whilst teaching a group of talented boys and girls, he entered the room with a jar full of flees, the jar had a top on it. They all watched as the fleas jumped up and hit the top over and over again.
As they all watch them jump and hit the top, they notice something interesting happen, the fleas continue to jump, but they no longer jumping high enough to hit the top.
Tao took the top off and though the fleas continue to jump, they didn't jump out of the jar. He told the students they won't jump out because they can't, and the reason is simple. They have conditioned themselves to jump just so high, but once they had conditioned themselves to jump just so high, that's all they can do.
The fleas, they fail to jump higher, because they were thinking they were doing all they can do.