Which of these men has been to your house?

Was it the same day as the other one or a different day?

Did Daddy go out to dinner with him or did they stay in the office?

Does Daddy say anybody’s name a lot on the phone at the moment?

That is it.

That is the entire tradecraft.

And I want to explain why it worked, because when I have described it to people, the response is nearly always that it is absurd — that no seven-year-old could possibly be a useful source of anything.

A seven-year-old is a superb source, for four reasons.