The word to show your teeth in Chinese is 呲牙 or 龇牙? I have seen 龇牙 in dictionaries but 呲牙 in like Wechat and other places. So do they have any differences or are they the same?
《现代汉语词典》 lists "呲" as a variant form of "龇".
According to 《现代汉语词典》, the official reading of "龇牙" is "zi1ya2", and so does "呲牙" (as a variant form of writing). However, "呲" has another reading "ci1". In my experience, the reading "ci1ya2" for the word is used so common that I only found the official reading when I looked it up in the dictionary. And when you input it with pinyin "ci1ya2" on a computer or phone, you get "呲牙" easily. This may be the reason that you see "呲牙" a lot.