The tongue is a part of Transformer anatomy sometimes found inside the mouth. Though Transformer tongues seem similar in structure and appearance to the tongues of humans, what function they serve on Transformers is uncertain. Though considering that Transformers appear to speak just like humans, using what appear to be lips and teeth to form phonemes, it's possible the Transformers who have tongues use them to help form those sounds. Whether Transformers also use tongues to taste, as humans do, is unknown. Or maybe they serve no purpose and somebody built them crazy.