Inverted Glass Of Water
To show one result of atmospheric pressure.
Glass, beaker of water and 3x5 card.
Fill a glass with water and lay a 3x5 card on the top. Holding the card and glass together, turn them upside down and release the card. Tilt the glass slightly. Atmospheric pressure holds the card onto the opening of the glass.
Hans Weltin, A Paradox, AJP 29, 711-712 (1961). Moti Nissani, Clifford L. Maier, and Norma Shifrin, A Guided Discovery Exercise for Introductory Physics Labs, TPT 32, 104-107 (1994).