Showing One Card (after everyone has folded)

[mntndrew]

Slightly different question, but if there's an actual showdown, though, you have to show down both cards, right?

Situation: P1 bets, P2 calls. P1 shows top pair but doesn't show kicker.

Or, O8, P1 bets, P2 calls, P1 says he doesn't have low and tables the two cards for his high hand.

You have to show all your cards at an actual showdown, don't you? Or can you do this if P2 mucks without contest if the shown cards are enough?