I'm confused. Is it straightener or straighteners?
A few of us thought "straightener" because it is a singular item however others have said "staighteners" because although it is 1 item, it has 2 hot irons. (Much like Scissors?) You wouldn't say... "Can I use you scissor?"
I guess we're trying to find out if you would say "Can I use yours straightener?" or "Can I use your straigh