for those trying to keep track, we are now of to physiology.. some retiles and other primitive animals lack cheeks and lips! or at best, have very small lips.
cheeks and lips help hold food in the mouth.. lips also protect your teeth. lips are skin, but not the same skin as on most of your body.. they don't sweat, and they don't have oil glands (or at least they don't have as many, that's why your lips chap much more easily than other skin, and why lack of vit. C shows up first there.)
Inside your mouth, (and at other body openings) you have a special skin, a mucos membrane.. this wet skin has special glands (salivia glands) that keep it wet. (ear wax has a similar function, as do tear, etc) .. some where in the back of your mouth, (and in your nose, and under you eyelids, and in your ear canal, the skin gives way to? viscera? (for want of a better name)i think there is a clear demarcation, just as your lips are clearly different in texture from your skin, but, i'll let on of the doctors correct all the details, (since what i know is, close to right, but.. i wouldn't get an A on the anatomy test..)
in any case, what we think of as a grin is a big smile, lips pulled back, and lots of teeth showing..
but reptiles don't have lips or cheeks.. and croc's mouths have an natural upturn, so they always are showing teeth, and the shape of their jaw, is such, they always seem to be smiling.
dogs have lips, and squirrels have lips and cheeks.. most (all?) mammals have lips and cheeks.. (or at least all primated do.. maybe not all mammals.. ) I am not sure--do we have a comparative biologiest here? who else has watched 1000 hours of NATURE and read enough to know.. i only have a good idea.)
