What is the difference between "such fun" and "so fun"?

Native speaker’s answer
Rebecca
The difference between "such fun" and "so fun" is that "such fun" is grammatically correct, and "so fun" is not. While "so fun" is not grammatically correct, it is more common for native English speakers to say than "such fun". The reason "so fun" is not grammatically correct is because "so" is an adverb and "fun" is a noun, and adverbs can't modify nouns.