Teeth

Green spittle not always included.

Teeth are a part of Transformer anatomy sometimes found inside the mouth, surrounding the tongue. Though Transformer teeth seem similar in structure and appearance to the teeth of humans, what function they serve on Transformers can be a subject of debate.

Humans use teeth primarily for breaking down the foods they consume into digestible portions, but Transformers fuel is mostly, if not entirely, liquid. Human teeth are also used to help form certain sounds in their language, in conjunction with the tongue. Transformers are capable of human speech, and sometimes are shown to have both teeth and a tongue, so it's possible their teeth may be used for communication. Two functions of teeth definitely shared between humans and Transformers are using them as a weapon and baring them to indicate hostility.

Transformers who have animal-based alternate modes tend to have teeth in those adopted forms. Most of these alt modes were acquired after contact with other planets such as Earth, so the inclusion of teeth in Transformer anatomy is often irrelevant to understanding why some Transformers have them.

The only medium which avoided Transformer teeth entirely was the original cartoon, in which the inside of the mouth was depicted as a solid dark gray. In all other media, teeth were either uniformly used (such as in Beast Wars) or their appearance varied by artist (such as in Marvel Comics).

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  Don Perlin gave Generation 1 Megatron teeth when he felt like it. I, Robot-Master! Gone But Not Forgotten!
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  Teeth are helpful in creating a variety of facial expressions. The Agenda, Part 3
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  One of Optimal Optimus' teeth were knocked out during a fight. Nemesis, Part 2
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  A vanishingly-brief moment of somebody actually having a facial expression in Cybertron. Ambush

From Teletraan I: The Transformers Wiki, a Wikia wiki.