If you are a girl, imagine for 2 minutes that you are a guy. For guys, remain a guy (unless it turns you on to imagine yourself a girl).
Consider this scenario: you're peeing at the urinal and someone comes to pee next to you. Instinctively, you look to your side and you happen to meet his eyes as well. He looks kinda familiar, but you don't remember him. You smile at each other and continue about your business.
Two seconds later you remember that he is the brother of your good friend. You haven't seen him for a long time, but you're pretty sure he remembers you (since he did smile back at you). What will you do? Exchange pleasantries and chat about what he has been up to while answering nature's call? Or wait until both of you are done peeing but risk appearing rude and "unthoughtful"?