How Did The Mouse Help The Lion In Aesop's Fable?

A lion caught a tiny mouse and was about to devour it when the mouse began to beg for mercy. It said, "If you will spare my life now, one day I promise I will save yours." The lion found this idea so funny that he let the mouse go, although of course he didn't believe that the little creature could ever really save his life. He thought no more of it until, long afterwards, he was caught by hunters and tied down with ropes. As he struggled to escape, he felt something tickling him; it was the mouse. It nibbled patiently through the ropes until the lion was free.
This fable is sometimes used to illustrate how apparently powerful people should never despise the contribution of the less powerful; or, more selfishly, that it pays to be nice to others, even if they don't appear useful, in case you ever need their help.