Question

what makes a coaster placed over the rim of a glass of water stick even when youinvert it?

Answer 1

its because of the pressure of the air that locks it tight, the air pressure keeps the water from falling out of the glass. The air pressure outside the glass is greater than the water inside the glass therefore it holds the coaster to glass

Answer 2

the atmospheric pressure outside to glass balances the pressure exerted by water from inside the glass. this is how it works