The famous Trolley dilemma

The "Trolley Dilemma" (or the "Trolley Problem") consists of a series of hypothetical scenarios developed by British philosopher Philippa Foot in 1967. Each scenario presents an extreme environment that tests the subject's ethical prowess. In 1985, American philosopher Judith Jarvis Thomson scrutinized and expanded on Foot's ideas in The Yale Law Journal.
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. Further ahead on the track, 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 similarly tied up on the side-track.
So 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.