Why doesn't the belle of some painter the wording and purpose of what one writes have smile?

Actually, these artistic model, if not expressional coma is pretty good, still want them to show pleasant smiles, too difficult really! In daily life, everybody knows, when taking a picture, a few seconds have smiling expression is no problem, if a few minutes, ten minutes, still maintain smile, not easy really, everybody has such feeling. So, the artistic model of painter the wording and purpose of what one writes, basically be a belle, without smile, and Leng Yan, also did not pass normally again, after all, they should maintain some kind of stance a few hours, it is 10 days of half moon even! And one the individual's smile, at most a few minutes caky! This shows, we see paint is medium usually, the belle does not have smile commonly, be to do not have method really! because such, cheat the smile of graceful Li Sha, just appear so precious!