dull vs sharpen

dull is an adjective but sharpen is not an adjective.

Word	Noun	Adjective	Verb	Adverb
dull	No	Yes	Yes	No
sharpen	No	No	Yes	No

dull (adjective)	sharpen (adjective)
not having a sharp edge or point
emitting or reflecting very little light
lacking in liveliness or animation
not keenly felt
darkened with overcast
(of business) not active or brisk
blunted in responsiveness or sensibility
being or made softer or less loud or clear
slow to learn or understand; lacking intellectual acuity
not clear and resonant; sounding as if striking with or against something relatively soft
(of color) very low in saturation; highly diluted
so lacking in interest as to cause mental weariness

dull (verb)	sharpen (verb)
become dull or lusterless in appearance; lose shine or brightness	become sharp or sharper
make less lively or vigorous	make crisp or more crisp and precise
become less interesting or attractive	raise the pitch of (musical notes)
make dull in appearance	put (an image) into focus
make dull or blunt	make (one's senses) more acute
deaden (a sound or noise), especially by wrapping	give a point to
make numb or insensitive	make sharp or sharper
	make (images or sounds) sharp or sharper