英语专业四级考试听力材料：How High Can You Jump?

　　Fleas trainers have observed a strange habit of fleas while training them.
　　Fleas are trained by putting them in a cardboard box with a top on it. The fleas will jump up and hit the top of the cardboard box over and over and over again. As you watch them jump and hit the lid, something very interesting becomes obvious. The fleas continue to jump, but they are no longer jumping high enough to hit the top.
　　When you take off the lid, the fleas continue to jump, but they will not jump out of the box. They will not jump out because they cannot jump out. Why? The reason is simple. They have conditioned themselves to jump just so high. Once they have conditioned themselves to jump just so high. Once they have conditioned themselves to jump just so high, that is all they can do!
　　Many times, people do the same thing. They restrict themselves and never reach their potential. Just like the fleas, they fail to jump higher, thinking they are doing al they can do.