Trying to claim pot with one card.

[bav]

If you are supposed to be showing first by the rules of the room and the guy is showing you one card, he's doing you a favor by showing out of turn. In such a case, if you want to keep your cards secret, just muck 'em and don't ask to see his other card. Demanding he show both cards to claim the pot when it's really YOU who should be showing both first is uncool.

If instead he is supposed to be showing first, just request he table both cards before you do a thing. No need for theatrics. Just a polite "it takes two cards to win" or "I'd like to see both cards, please".