Transformation is the franchise's signature trick, but how does it work inside the fiction? Different series explain it differently, yet a few ideas recur most often.
The transformation cog
In many modern comics and series transformation is driven by a special organ — the T-cog. It rebuilds the body from one form to another. Losing a T-cog means being stuck in one form, and for a Transformer that is a serious injury.
Scanning Earth vehicles
On arriving on Earth, Transformers scan local machines and gain a new alt-mode so they can hide among humans. That's why in the films Bumblebee first looks like an old car and later picks a new model.
Why sizes don't add up
Fans noticed long ago that Transformers often change size when they transform — the best-known example being Megatron, who in the classic cartoon became a pistol in Starscream's hand. This is explained by 'mass shifting', a hand-wave technology for moving mass. The real reason is simpler: that's what the toys looked like, and the cartoon followed them. More in the article on mass shifting.
The sound everyone knows
Worth a mention too is the famous transformation sound from the 1984 cartoon — a mechanical whir that became the franchise's audio signature.
