Young-Earth creationist Ken Ham proudly teaches children to ask 'were you there?' of anyone making statements about evolution, the age of the Earth, or indeed of any scientific statements involving long periods of time. This isn't a genuine question, as someone who's been coached by Ham & his ilk will feel that they already know the answer, so they're not asking out of a desire to learn more. On Pharyngula, PZ has written a beautiful post - in the form of a letter to one such child - on why it's so much more interesting, and rewarding, to ask 'how do you know?' instead.