and what about word, that are normally plural, say pajamas, alway worn, and bought in the plural, but if you are a teen age girl, you have a pajama party.. when you want to express the plural (lots of pajamas, at a party) the word become singular...

or am i wrong, did pajamas come to us from the hindu with the s as part of the word?--I think not, because pajama is also used for half a pajamas-- (she wore his pajama top as a nightshirt...)