Try This Yourself!

Air pressure will support the water in an overturned glass just as it supports the mercury in a barometer. Submerge a glass in a sink full of water, allowing the glass to fill. Then lift it (bottom-up) slowly out of the water. The glass remains full as long as its lip stays below the surface of the water in the sink. The air pressure against the water in the sink prevents the water from draining out of the glass.