In an experiment, a scientist placed a number of fleas in a glass jar. They quickly jumped out. He then put the fleas back into the jar and placed a glass lid over the top. The fleas began jumping and hitting the glass lid, falling back down into the jar. After a while, the fleas, conditioned to the presence of the glass lid, began jumping slightly below the glass lid so as not to hit it. The scientist then removed the glass lid as it was no longer needed to keep the fleas in the jar. The fleas have learned to limit themselves from jumping beyond the height of the lid even if the lid was removed as they have been conditioned to the fact that they cannot escape from the jar.