6 of One, Half-dozen of Another

Recently, when I went to McDonald's I saw on the menu that you could have an order of 6, 9 or 12 Chicken McNuggets. I asked for a half dozen nuggets.

'We don't have half dozen nuggets,' said the teenager at the counter.

'You don't?' I replied.

'We only have six, nine, or twelve,' was the reply.

'So I can't order a half dozen nuggets, but I can order six?'

'That's right.'

So I shook my head and ordered six McNuggets.