Is day a proper noun or common

the jewellery thief was as cunning as a fox.

explanation:

"as cunning as a fox" is the simile, because it compares two things using the words "like" or "as" to do it.

it can be both; depending on the situation at hand.

explanation:

let's just say i get in to a confrontation with someone, and they say "ur mom", it could be seen as an insult. or depending on the relationship or comfortability with the person it may be seen as a joke. the distinct line is how the chemistry behind the confrontation.