What would you do?
Imagine a tram hurtling out of control towards five pedestrians. If you could flick a lever to switch the tram on to a track with just one person, is it permissible to kill one to save five?
Then think of yourself on a bridge above the track. Now the only way to stop the tram is to push down a heavy object – and there is one beside you, in the form of a fat stranger.
Is there a difference in these two situations? What is the difference?