From another of my teaching readings

Start with the students rather than the discipline. Every year more than 700 students crowd into Sandel’s classroom at Harvard to take his course on justice, in which he asks them to imagine the following scenario: You are the driver of a runaway trolley car that is approaching five men who are working on the track. You cannot stop the train, and it seems destined to run over the men and kill them. As you speed down the track toward this waiting tragedy, you notice a side track where you can steer the trolley car if you choose to do so. The only problem is that one man is working on that track and the train will undoubtedly kill him if it goes that way. What would you choose to do, he asks the students? Do you turn the car onto the side track, killing one person but saving five others? What would be most just and why? Often the students have no difficulty deciding that they would take out the one life to save the five others.

Sandel then introduces a wrinkle to the story. Suppose, he says, that you are not on the train but standing on an overpass watching it speed toward the five workers. As you watch this disaster in the making, you notice a large man standing next to you, also peering over the railing of the overpass. You quickly calculate that if you push this person over the railing, he will land on the track in front of the train. He will die, but his body will stop the train, saving five lives. Would it be just to give that person a shove?

In case you forgot what philosophy class was like.