serious vs fun

serious is not a noun while fun is a noun.

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

serious (noun)	fun (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)

serious (adjective)	fun (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