Whats this phenomenon called?

Whats this phenomenon called? So say you're blocking/stuttering trying to get out a word, somehow another person says the word and you can finally say it... Example, John is blocking on completinga sentence, "Hey doe, do you want to go have....[pause].... Doe knows what he is trying to say and says, "what? dinner?" John replies "Yes Dinner!" Why does this happen and what is it called? Simply, you can't say a word until someone else says it out load?