Watch television for a couple of hours this evening and, along with the entertainment of your choice, you’re also going to be exposed to a large array of commercials. Many of these will be instantly forgettable: it doesn’t matter how many times you see them you probably don’t remember them or what brand they’re trying to sell. Others, however, will catch your imagination and pull on your emotions, to such an extent that you may find yourself remembering them and even sharing and talking about them with others later in the day. But what is...