Dictionaries say that the word, all -among other functions- is an adjective. I seem not to understand this. I was taught that to make sure whether a word is adjective one can make a sentence with [the specific] noun + to be + [the specific] adjective. i.e. “blue sky”, “sky is blue.” This formula seems to function in all the adjectives except “all.” Can anyone explain why the English dictionaries call “all” an adjective? I have looked up many examples, but it didn’t help.