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.
What's the difference between a smile （微笑） and a grimace (龇牙咧嘴)?
There is also 露出 for show your fangs!
Character decomposition 字形分解 [?]: Compound 齜
from teeth-chi 齿齒 chǐ and
phonetic this-ci 此 cǐ
They say it was only pronounced zī, but maybe some people always said cǐ or cī, because of the phonetic component.