I didn’t say the guess was always right, nor did I say the listener interrupts the speaker and never hears the intended finale.
But people who can track a developing argument and complete it in their own minds are more often correct than not.
Which is kind of the whole point.
1) Animals that bear live young and nourish them with material produced in their bodies are usually mammals.
2) Whales bear live young and feed them milk
3) So a whale …
“… is a mammal!! What do I win?”