Anyone who has ever had a French lesson will know the phrase "bon appétit".
And even if you haven't you'll be familiar with it because it's one of the most commonly used French expressions in English too.
But woe betide you if you say it to anyone who's a stickler for good manners in France.
Apparently, despite the fact that everyone from French children to foreign learners alike are told that it is polite to use the phrase at meal times, we shouldn't actually say "Bon Appétit" before we tuck in to our food.
A quick Google search in French will bring up an array of results, including blog posts discussing why these two little words when used together are not fit for purpose.
One blogger writes: "Don't wish me a 'bon appetit'".
Another explains "Why you should never say 'Bon appetit'" (see below).
Astoundingly, the tradition of not saying "bon appétit", often shortened to "bon app", dates back to the Middle Ages when people started to see food and dining as more than just a question of survival.
When cuisine began to be seen as a form of pleasure, people decided it was wrong to draw attention to the physiological act of digesting which is highlighted by the phrase "Bon appétit".
One blogger goes as far as to suggest it's akin to saying, "Enjoy your good intestinal transit".
And apparently to some, the person saying it will be considered boorish and uncouth.
French expert on good manners, Jérémy Côme told BFM TV that there is no question about whether or not you should say "Bon appétit" because the expression means "good digestion".
"We do not say 'Bon appétit', said Côme. "It's too intimate, it's the phrase to ban, 'Bon déjeuner' (enjoy your lunch), 'Bon diner' but not Bon Appétit."
Another good way of replacing the phrase is by saying "Bonne degustation" says Côme. In French "degustation" means "to taste" or "savour" so the focus is on the pleasure of eating or indeed drinking.
Another reason for the phrase being considered unfit for purpose is that it suggests that you need courage to eat the meal in front of you and if said by guests at a dinner party it could be considered insulting to the host.
However there are some dissenters who say that "Bon appetit" is not considered rude anymore.
"It's a generational thing and while strictly you're not supposed to do it, people of my generation say it all the time," Clotilde Dusoulier, Paris food writer and author of the blog Chocolate & Zucchini told The Local.
"Manners have loosened up from previous generations. Rules around meals are less codified and dining is more about enjoyment. It's also seen as a signal that you can begin the meal," Dusoulier, who is aged in her 30s, said.
"It used to be seen as informal but the younger generation in France is all about informality around food. We're doing away with a lot of the old rules of etiquette," she added.
French language expert Camille Chevalier-Karfis and founder of the language site French Today says it's all a question of setting.
"In France as in many countries, there are different levels of etiquette. If invited for supper at the Queen's, you will not behave the same way as you do when sharing a casual meals with our friends, and things will be different yet again if you were invited to your boss's house," she told The Local.
"So, saying 'bon appétit' may not be acceptable in the most formal of situations... Whether it is used appropriately or not, the fact is that it is otherwise extremely common to say 'bon appétit' before starting your meal in France."
One blogger goes as far as to suggest it's akin to saying,"Enjoy your good intestinal transit".