Supposedly a true story:
One time a clergyman was visiting a circus and saw an elephant tied to a little stake in the ground, and not even attempting to get away. He asked why the elephant didn't pull it out, since it would be easy, and this is what he was told:
"You see, when he was a baby we tied him to a stake. As you would expect, he pulled and pulled trying to get away, but couldn't. So now, he KNOWS he CAN'T pull up the stake. So HE DOESN'T EVEN TRY anymore. That's how we train them."