Why no singular for tongs?

The noun 'tongs' is one of a group of nouns sometimes called 'binary nouns', a Word for things that require two halves to form a whole object. The binary nouns, like tongs are both singular and plural because they are a shortened form for 'a pair of'. Some examples:

• a pair of tongs; two pairs of tongs
• a pair of scissors; two pairs of scissors
• a pair of tweezers; two pairs of tweezers
• a pair of pants; two pairs of pants
• a pair of shorts; two pairs of shorts
• a pair of pajamas; two pairs of pajamas
• a pair of binoculars; four pairs of binoculars
• a pair of glasses; two pairs of glasses