You enter a room with two cages. One contains a friend, who is clearly distressed. The other contains a bar of chocolate, which clearly isn’t. What do you do? While a few people would probably go for the chocolate first (and you know who you are), most would choose to free the friend. And so, it seems, would a rat.
Link to original article: http://www.sciencemag.org/content/334/6061/1427