THE BEST RIDDLE EVER

I am often asked what my favorite riddle is. Actually, that's a lie. No one has ever asked me that. But I do have a favorite, and here it is. It was written by George Canning, a British statesman (1770-1827). Riddles of this type are also called enigmas.

A word there is of plural number,
Foe to ease and tranquil slumber;
With any other word you take,
To add an s would plural make.
But if you add an s to this,
How strange the metamorphosis:
What plural was, is plural now no more,
And sweet, what bitter was before.

What are the two words?

Answer:

CARES - CARESS