[youtube=http://www.youtube.com/watch?v=U9cGdRNMdQQ&w=700]  

Smile! Ron Gutman reviews a raft of studies about smiling, and reveals some surprising results. Did you know your smile can be a predictor of how long you'll live -- and that a simple smile has a measurable effect on your overall well-being? Prepare to flex a few facial muscles as you learn more about this evolutionarily contagious behaviour.

Try this BBC quiz on spotting fake smiles.