Glass = a usually transparent or translucent material consisting of a mixture of silicates; something made of glass such as tumbler , mirror etc.

• She drank a glass of water.
• He drank two glasses of water.
• The waiter filled our glasses with water.
• A drinking container made out of glass .
• A glass of milk.

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Glasses = A pair of lenses set in a frame resting on the nose and ears, used to assist defective eyesight.

• He began to use reading glasses when he felt strain in reading.
• I keep a second pair of glasses(NOT glass) in my office.
• Those glasses are only for reading.
• I could read without glasses last year.
• That glass is mine.( incorrect)
• That pair of glasses is mine.(correct)
• Those glasses are mine.(correct)

Note :- The same rule applies in the case of trousers, binoculars etc.

• She bought a pair of shoes. ( But, in the case of shoes you may come across the following words where ‘shoe’ is used alone. (shoe box, shoe brush, shoe cream, shoe leather, shoe polish, shoe shop, shoe repairer, horse shoe etc., maybe, due to the fact that it is separable into two pieces)