Funny story that. "Scissors" is not a plural, but is actually already a singular noun. There is no such thing as a "scissor" it's called "a pair of scissors" due to there being two matching blades. Which is very awkward when you think about it, because it's like saying "a pair of sock", but because English sucks, it's still technically correct to say coming from middle English usage. It's referred to as a "plurale tantum".
