It's one thing to be rude on purpose. While it's never an admirable activity, some people truly have elevated rudeness to an art form and have been rewarded with riches and their own television shows. (I don't think it would be rude to point a finger here at Joan Rivers and Simon Cowell.)

Where rudeness is particularly sad and never rewarded financially is where it's unintended.

Take the Ugly American syndrome, which happens when Americans go overseas and are loud, badly dressed, and disrespectful of local customs. These people don't mean to offend when they strut into ancient temples wearing dimple-revealing short-shorts, but offend they do.

Likewise, I have to think that Carson Daly doesn't mean to say, "Your wife has cheated on you and given you the horns of a cuckold," to all those MTV viewers who watch him faithfully when he makes "horn hands." MTV has such a large audience, and so many wives to corrupt, that I have to give him the benefit of the doubt here, despite his reputation as a ladies' man.

More likely, he's using the "hang loose" meaning, or possibly even the American Sign Language gesture that means, "I love you" (and have no intention of trying to seduce your wife).

Nonetheless, in many parts of the world and depending on how the horn fingers are pointing, the gesture is quite an insult, ranging from the wife slur to a more generic curse upon you.

Similarly, George Bush, Sr., did not mean to say, "Up yours!" to Australians when he visited there in the early '90s, but when he flashed the victory sign, he pointed his palm in the wrong direction and accidentally made the news that way. It's an honest mistake among conservatives, apparently. Margaret Thatcher did it too and the Associated Press caught it on film.

Even the "thumbs up" gesture Bill Clinton often employed during his presidential speeches is offensive to some. In Greece and Sardinia, it means something that can only be politely said to one of the local olives: Get stuffed.

One could make the argument that if a "thumbs up" is offensive to some people, then it's impossible to be universally polite without tying your hands behind your back.

Maybe so. You could fill a dictionary with the vocabulary of gestures that can easily be made with the hands and face, a staggering amount of which are insulting or obscene. (In fact, there is a dictionary like this--The Dictionary of Worldwide Gestures, by Betty J. Bauml and Franz H. Bauml.)

But just knowing what's rude and why is a big step in avoiding the embarrassment of accidentally insulting people, which is why I've selected the Top 4 Potentially Rude Things You Can Do Without Even Opening Your Mouth.