Wikipedia explains the trolley problem:

The trolley problem is a thought experiment in ethics. The general form of the problem is this: There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. 

You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. 

You have two options: 

1. Do nothing, and the trolley kills the five people on the main track. 
2. Pull the lever, diverting the trolley onto the side track where it will kill one person. 

Which is the most ethical choice?

That entry says the problem was introduced by Philippa Foot in 1967, but I did not come across it when I studied moral philosophy a dozen years later.

There were similar puzzles around, which were designed to embarrass utilitarians. Their theories obliged them to make one choice while, it was argued, our moral intuitions told us that the other choice was right.

I was never sure how much those moral intuitions were worth in the abstract. Do we really know what we would feel in an extreme situation? Sacrificing a few troops to save many more must quickly come to feel moral in time of war.

Anyway, I hope this young man has solved the trolley problem and that ethics will take a more interesting turn as a result.