"It is well-known that those who have charge of young infants, that it is difficult to feel sure when certain movements about their mouths are really expressive; that is when they really smile. Hence I carefully watched my own infants. One of them at the age of forty-five days, and being in a happy frame of mind, smiled... I observed the same thing on the following day: but on the third day the child was not quite well and there was no trace of a smile, and this renders it probable that the previous smiles were real."