A mouth is one of the eight known orifices found in all mammals. Ten points if you can name John Travolta.
edit Parts of the Mouth
These are for lipstick (as the name suggests). They act as a kind of buffer-zone between the face and the mouth, to prevent unnecessary hostilities between the two.
While the tongue can be used for lipstick too, it is mostly used for talking and chewing. Owing to its busy schedule of talking and chewing, the tongue is stinging ripped, and is in fact the strongest muscle in the body. This makes it one of the most important parts of the mouth, especially when offering a loved one an oral service.
Teeth are hard and used for biting things. Things they are often used to bite are; food, tape, frozen milk, your mum, and chewing gum.
The tonsils were installed as a revenue generating means by surgeons. Their scientific title is "Dingalum Minoris," which translates into English as "the Unimportant Danglies." There are, or course, no prizes for guessing what part of the anatomy is the "Dingalum Majoris." The least said about the worthless tissue that is the tonsils, the better.
The cheeks are as vital a part of the mouth and its workings as any other. Though oft unappreciated, it is evident in every process that the human does with his or her mouth, be it kissing or sucking a lollypop. Cheeks are a must for any occasion.
The mouth has many functions. These are as diverse as they are useful and interesting.
The mouth is the entry point for all food into the body. It's good at this because it opens onto the outside (so food can be inserted) and it also opens onto the stomach. As such, it kind of acts like a laundry shoot for food. It is also useful as it can be used to break food up. The teeth are especially useful here, but the tongue is also good for mashing. Imagine trying to insert a cracker into your stomach without a mouth, and you'll see why we need mouths. Your tongue (mine too, actually) also does the important job of tasting. If you couldn't taste, you wouldn't know that semen is really salty and should be spat out.
No one is really sure how talking works, but everyone knows that it comes out of your mouth. Many women are working around the clock to elucidate the mechanism, or at least the effect, of excessive talking. Rumour has it that some vocal cords do some vibrating, the tongue does some dancing and lips give words a kiss on the way out. Rumour also has it that your mum is good in bed, though.
Spitting is a critical defence mechanism, not just for camels, but for all species with mouths. In a dire situation, such as one in which you are tied up with a bad person of another ethnicity close-by, spitting can be your only possible course of action. Unfortunately, most saliva is only mildly acidic, and will not corrode spaceship walls.
edit Spitting (alternative version)
It supposedly doesn't taste good. But with the right kind of diet, the flavor may change. Studies show eating something sweet or nutritious like strawberries or chocolate will in fact make it sweet. Smoking will make it bitter and drinking alcoholic substance will make it sour. 9 out of 10 doctors agree that a banana split with crack and tobacco sprinkled on top of it is the surest bet.