fun vs serious

fun is a noun but serious is not a noun.

Word	Noun	Adjective	Verb	Adverb
fun	Yes	No	No	No
serious	No	Yes	No	No

fun (noun)	serious (noun)
activities that are enjoyable or amusing
violent and excited activity
a disposition to find (or make) causes for amusement
verbal wit or mockery (often at another's expense but not to be taken seriously)

fun (adjective)	serious (adjective)
	concerned with work or important matters rather than play or trivialities
	completely lacking in playfulness
	causing fear or anxiety by threatening great harm
	requiring effort or concentration; complex and not easy to answer or solve
	of great consequence
	appealing to the mind