If you want to pretend to be an all-knowing psychic, here is something you can say that will always be right:

"One of your ancestors made bootleg home brew during Prohibition." This is a 100% sure hit. You can get even more specific than that, as every family has the same set of Prohibition stories:

The beer produced was terrible, but it had alcohol in it and it fizzed, so that is all that mattered.

On warm summer nights, you could hear the bottles exploding in the basement.

One family member made home brew that was so good that when Prohibition ended he kept making it.

Oddly enough, there was never any problem with the police.

--Come to think of it, I'd be interesting in seeing any family Prohibition stories here.