Joke Collection Website - Public benefit messages - Why does an unread short message in English add S?

Why does an unread short message in English add S?

in "an unread message", if an unread message is followed by an "s", it must be a sick sentence.

if you want to translate SMS, which means Short Messaging Service, the "s" behind it is not a plural s, so it is not easy to type. Please adopt it.