Let's see how good you are at it http://www.bbc.co.uk/science/humanbody/mind/surveys/smiles/ I got 16/20, however I wasn't able to watch 4 clips so I had to guess. That might have had a negative or positive effect on the score.
There was a thread about this not too long ago. I can't remember what I got. I think I got 16 or 17 right.
im not sure how many i got, i went back to answer its 'which was most helpful' question, but it forgot all of my answers and i couldnt be bothered doing it again.