This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
A little word of doubtful number,
a foe to rest and peaceful slumber.
If you add an "s" to this,
great is the metamorphosis.
Plural is plural now no more,
and sweet what bitter was before.
What am I?
cares -> caress