There are two types of smile: fake and genuine.
The muscles which control the cheeks and the muscles which control the eyes get switched on and off by different parts of your brain.
The parts of your brain which make up the conscious, in-control you are the same parts which can yank at your mouth and fake a smile.
Real smiles include the muscles around the eyes, which are harder to control and thus are less likely to squinch up when you are committing grin fraud.
Do you think you can easily spot the difference? (Research shows that most of us – 90% according to one study – think we can. The reality is quite different.)