Fill a glass with water. Use a rubber band to hold a handkerchief or bandanna tightly over the top. Over a sink, quickly turn the glass upside down. The water should stay in the glass.
Use a finger to press upward gently on the handkerchief. The pressure will cause water to trickle out and air to flow in, with bubbles rising through the water.
What keeps the water from running out of the glass?
Photo by Guy Cali Associates, Inc.