Shep's Shibboleths (2)

A couple of times, I have told Shep he can't have two items he requests at the same time. I say, "You can't have both right now, so choose one. Do you want x or y? Which one do you want?" He chooses x, but a couple times he's changed his mind after a few minutes. Instead of requesting y directly, though, he says, "Ask 'Which one?'!" That one took me a while to work out.