One of the demands that models receive during photoshoots is to obtain a certain pose or expression/mood. Often they are asked to stare at the camera with a serious face, maybe that looks attractive but imagine having to do that for hours during a photoshoot, it gets tiring. Sometimes it's just better to not be so serious and give the model a break and tell her: show me the funniest face you can make!
Many years ago during a long photoshoot I noticed that this model got tired of being serious so I asked her to have a break, this is the result: