Why? Pair of Scissors?

The other day I said "a pair of scissors" and thought about the phrase for the first time. Why use the word "pair?" I found a variety of explanations, some of which made more sense than others.

When I looked this up I came across a whole list of terms that are plural but refer to one item. The word "pair" is used with many of them. Examples: 

  binoculars

  goggles

  jeans

  pants/trousers 

  shorts

  tights

Here I learned that a plural noun that "refuses to be singular," such as scissors, is a "defective" noun.  (I never knew there was such a thing.)

Check out this site for interesting information and explanations.