In a recent tournament I raised with 67o in an unopened pot from the cut-off in an effort to pick up the blinds/antes.
To my dismay, I got called by the button and the small blind.
To my relief, the flop came 3d4d5c. SB checks, I bet 1/2 pot, BTN folds, SB calls.
Turn comes 4s, SM jams.
I think about it for a minute, he would have raised two pair or a set on the flop because of the flush draw, look up his stack, determine how much I'd have left if I'm getting my money in bad (drawing dead...), and decide a call is correct. He shows 6c8c for an open ended straight draw, a 3-outer to beat me. He whiffs and leaves the tournament.
The players at the table were surprised to see I raised with 67o, surprised I bet when I flop the nuts (even though I was trying to not give away a free diamond), and that I call on a paired board with a card to come. I don't answer any of them with honest answers, because I swore off giving poker lessons at the poker table.
Had SB bet half his stack, I would have raised him. Had he folded, I would have mucked my hand.
They got enough information from the call, which is going to effect my game (steal less for a few orbits? check the nuts if the board is dry?)
Why would I give them that information for free?
On the other hand, at a recent cash game, I had 3h5h and decided to play to mix it up a little bit. I whiff on the flop, which has two spades on it, but decide to call a bet. The turn comes another spade, the original bettor bets, I raise 3x his bet, he makes an obvious "damnit, stupid turn face" and calls. I fire $120 into the pot on the river after his check (the river was another spade) and he folds.
I show my bluff, with the intention of the next time I do that I'll have the nuts.