Raise a filled glass of water above the waterline in a body of water, but with its mouth b...

Raise a filled glass of water above the waterline in a body of water, but with its mouth beneath the surface. Why does the water not run out? How tall would a glass have to be before water began to run out? (You won’t be able to do this indoors unless you have a ceiling that is at least 10.3-m higher than the waterline.)