In this situation a call is in order. The river card say finished his flush, now if the player wants to trap, and check, you get to check along for free, but even if he bets,, a call from me he would get, just to find out what, and how he plays. If not too exspensive, this tactic can gain you two things. 1) Players get to see you will call with a med hand, and 2) You gain info on the other player, did he bet that flush on the river, did he check it, hoping you would bet it, was he bluffing etc. One hand is not out of the question to let the other players know, you will not just lay down in the corner at a bet. You gain even if you have second best cards, know what I mean!! Keep in mind, that if at a casino, you will be able to gain even more info, how is the player looking to you, is he nervous, does he look uneasy etc. Very first hand, and you did have a pair, and you let the other players know, you like to see a hand play out! Set an image, but just an image of course!