Two very short friends, John and Jack, were living together in an apartment. Since they used to lose their apartment key very often, they decided to leave it on top of the door frame when they leave home. In order to reach the key, John was climbing on Jack's shoulders and thus taking it down from the frame. However, John was the taller and the heavier guy of the two. Why didn't Jack climb on his shoulders instead?
The reason is that as being taller, John also had longer arms. If Jack climbed on John's shoulders, he wouldn't have reached the key.