It's kind of a strange when you think about it, but rudeness is 100 percent a cultural construct. Anything that you can think of that's considered rude in American culture, for example, might be seen as perfectly fine (and maybe even positive) in another place.

What you have to apologize for varies from culture to culture. The key, of course, is knowing what's acceptable where you happen to be. So before you head out on your next trip, check out what the local customs are. Here are a few of some of the most common taboos from around the world that are different from what we do in the United States.

1. Tipping varies from country to country. While it's considered rude not to tip for myriad services in the U.S., from servers at restaurants to the dryer at the car wash to a cab driver, in Japan it's considered rude to tip. That's because great service is the expected norm, and tipping shows that you thought you might not get the best, which is insulting. In most European countries, the Middle East and Australia, tipping is more of a token than an expectation, since it doesn't form the base of the salary of the person who's assisting you. So it's still nice to do in these places, but smaller amounts are appropriate, though you won't be insulting anyone in these locations if you tip.

2. Opening gifts: In the U.S., it's generally good manners to open a gift that has been received in front of the person who has given it (and to express joy and thank the person who has given it to you then and there). But in China, India, and other countries in that region, one opens presents in private. Opening gifts in public is seen as tacky and a sign of a lack of self-control because the person receiving a gift is supposed to build up anticipation for the present. In China, you never, ever want to give an unwrapped gift; that's considered incredibly rude.

3. Using your left hand: You may have heard that in some parts of the world, the left hand is seen as unclean. While that custom has sensible origins — the left hand would be used to do dirty things related to urination and defecation and the right hand was used to eat — it's no longer true that the left hand is used this way because modern toilet facilities have become more common. But the taboo remains in many countries, including the Middle East, parts of Africa and India. In Ghana, even pointing, gesturing or handling items with the left hand is frowned upon. And in those places, you should never shake hands with your left, even if you're a leftie.

4. Public nose-blowing: In the U.S., honking one out with friends and in public is considered acceptable as long as you use a tissue or handkerchief. But in Saudi Arabia, France, China and Japan, blowing one's nose in public is considered disgusting. In China and Japan, using a handkerchief rather than disposable tissues is also looked down upon — the Japanese word hanakuso means "nose waste," which gives you a good idea of how they feel about it.

5. Chitchat is not appropriate in some places, but visitors can be ignorant of such unspoken rules. Talking in saunas in Finland and Sweden, gabbing in temples in Thailand (and churches in Europe), or chitchatting over food in some places in Africa, and China is verboten — the latter because it shows a lack of respect for the cook and the food. If you're not sure, look around and do what the locals are doing; if they're keeping mum, you should do the same.

6. Wearing shoes indoors is unpopular in many places, and removing them is a habit that's on the increase around the world as people seek to keep bacteria and dirt that can be tracked in on shoes out of the house. Homes with shoes-off policies had 60 percent less lead dust inside, for example. It's also easier to keep a home clean if shoes are removed. But in some places in the world, it's considered not only health-conscious to remove shoes a the door, but simply the right thing to do, and it's a custom that goes far back in history. In places as varied as Sweden and most of Scandinavia; China; Korea; most Pacific islands, including Hawaii; Thailand; Turkey; India; and much of the Middle East, shoes are not worn in the home. In some cultures, your host might provide slippers or flip-flops to wear instead. This practice is also becoming more common in American cities, especially New York and Boston, and in Europe and the United Kingdom. When in doubt, just ask your host if you should doff your shoes. (And keep your eyes open: One dead giveaway is a row of shoes near the doorway!)

7. Showing the bottom of your feet or shoes: Speaking of feet, showing the bottom of them — or the bottom of your shoes — in most Middle Eastern countries is taboo. The idea is that feet are generally unclean, especially the bottoms of them, so showing them to someone is an insult. If course, if you're not used to concealing the bottoms of your feet, you might do so inadvertently. If you do, just apologize; we all make mistakes. An easy way to avoid doing this is to never put your feet up on furniture or cross your knee while you are seated.

8. Signing off emails with hugs or kisses: Xx's and oo's, which are fairly common in the U.S. and U.K., can be seen as too intimate or forward by people in many places in the world. Stick to more formal sign offs until you know someone very well.

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Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.