Riddle Fun

Inspired by Lynn's post of yesterday, I thought I'd share my favorite riddle with y'all.  I heard it many years ago on Car Talk on NPR.  This was one of their Puzzlers. 
Here goes:

There's a word in the English language which ends in "s" and is masculine plural.  When you add an "s," it turns into feminine singular.  What is it?

As always, I moderate all comments.  I won't approve any until tomorrow night.  That should give you some time to think...