Most people think AI looks things up. It doesn't, not by default.
A hallucination is when the model generates information that sounds true but isn't.
It's pulling from everything it trained on, then extrapolating and filling in the blanks. When a blank gets filled with something that was never real, you get a fact that reads exactly like every correct fact around it. Same tone, same confidence, zero warning.
That's why "it sounded right" is a trap. Sounding right is the one thing it's always good at, whether the answer is true or invented.
If you had to explain a hallucination to a teammate in one sentence, what would you say?