Can Anyone Remember The Answer To This Riddle?

[ladybirder]

Neither my brother or I can remember and it's killing us.
A NOUN THERE IS OF PLURAL NUMBER
OPPOSED TO PEACE AND TRANQUIL SLUMBER
NOW ANY OTHER NOUN YOU TAKE
WHEN ADDING "S" YOU PLURAL MAKE
BUT WHEN YOU ADD AN "S" TO THIS
STRANGE IS THE METAMORPHOSIS
PLURAL NOW IS PLURAL NO MORE
AND SWEET WHAT WAS BITTER BEFORE

[jno]

cares

[mallyh]

princes .....princess?

[mallyh]

i missed out... prince princes princess

[elliemay1]

seen as 'cares' add S to become 'caress'

[mallyh]

i was trying to find a word to make plural and that's all i came up with didn't have a clue why lol x