"Say cheese" is an instruction used by photographers who want their subject to smile. By saying "cheese", most people form their mouths into what appears to be a smile-like shape.
As this practice became ingrained into modern western culture, it has taken on the simple role of a final warning before a photograph is taken. Often subjects will simply use the phrase "say cheese" as a cue to enter their final pose and to smile, neglecting to actually say "cheese".
Over the years, many other words have been used in place of cheese. For comedic effect, a photographer might say "Say ______" filling the blank with a word relevant to the event or action he is photographing (for example, "Say wipe out" during a seaside photograph). Other times a person will fill the blank with an absurdist or seemingly arbitrary word of their choice.
In the late 19th century, different aesthetic and behavioral norms required keeping the mouth small, which led to photographers using the phrase "say prunes".
In different languages and cultures
Perhaps because of strong Western influence, especially in the realm of photography, and perhaps because of increased numbers of Western visitors after photographic equipment became widely available, the phrase "Say cheese" has also entered into the Japanese language. However, the word "say" is almost always dropped from the phrase, resulting in the phrase simply being "Cheese." This is usually pronounced in Japanese (and written in katakana) as "chiizu" (チーズ).
Other languages have adopted this method, albeit with different words that sound similar to cheese to get the desired effect of shaping the mouth to form a smile.
Bulgaria: "Zele" ("Cabbage")
Brazil: "Olha o passarinho" ("Look at the little bird") or "Digam 'X'" ("Say 'X'") (the name of the letter "X" in Portuguese (/ʃis/) sounds a lot like the English word "cheese").
China: 茄子 (qie2zi), meaning "eggplant". The pronunciation of this word is notably similar to that of the English word "cheese". In Hong Kong, the phrase is "一,二,三" ("yat yi saam") meaning "1, 2, 3."
Croatia: "ptičica" ("little bird")
Czech Republic: "sýr" ("cheese")
Denmark: "Sig appelsin" ("Say orange")
Finland: "Muikku", a species of fish known in English as the vendace.
France and other French-speaking countries: "ouistiti" ("marmoset")
Germany: Food-related words like "Spaghetti", "Käsekuchen" (cheesecake), or "Wurst" are used, mainly to make children laugh for the picture.
Hungary: "Itt repül a kis madár" ("Here flies the little bird"). The English word "cheese" is also used, mostly by younger people.
India: "paneer" (Hindi: पनीर)
Italy: "sorridi" ("smile")
Morocco: "Khbiz" ("bread")
Iran: "سیب" (Saib), meaning "Apple."
Israel: "תגידו צ'יז" (tagidu tshiz), meaning "say cheese".
Japan: "Sei, No..." ("Ready, Set..."). Also チーズ (chīzu), meaning "cheese", is used.
Vietnam: "2... 3... Cười lên nào!" ("2... 3... Smile!"). And sometimes "i..i..i...." (pronounced like the name of the letter "E" in English).
Korea: "kimchi"
Most Latin American countries: "Diga 'whiskey'" ("Say 'whiskey'").
Nigeria: Many photographers prompt the subjects of their photographs to say "cheese" at the count of three
Russia: The English word "cheese", or sometimes the Russian word "сыр" (pronounced seer) which means "cheese". Also "Скажи изюм" (pronounced Skazhi izyum), meaning "Say raisins" (used as the title of a 1983 novel by Vasily Aksyonov.
Serbia: "птичица" ("Little bird")
Slovakia: "syr" ("cheese")
Spain: "di/decid patata" ("say potato"). Also, "mirar al pajarito" ("look at the birdie"), intended to make people look directly at the camera.
Sweden: "Säg omelett" ("Say omelette")
Turkey: "Peynir" ("cheese")
