How to make to make a glass stick to a tray
How can you turn an ordinary glass into a suction device?
Put a napkin on a flat tray and moisten it with water. Place a candle in the center of the napkin and light it. Cover the candle with a glass and press down hard to ensure a good seal. When the candle goes out, the glass will stick to the tray so securely that you’ll be able to use it to lift the weight of several fruits!
The candle flame heats the surrounding air. When covered by the glass, the temperature of this air is already above that of the ambient air. The amount of air inside the glass doesn't change, and its initial pressure is equal to atmospheric. As the candle burns, it consumes oxygen, and after a while the concentration of oxygen inside the glass is no longer sufficient to sustain combustion. Though the candle continues to heat the air, it will go out before the pressure inside the glass rises enough to cause any visible effects. After the candle goes out, the gas inside the glass cools to ambient temperature; meanwhile, its pressure drops, becoming much lower than that of the air outside the glass. This pressure difference causes the glass and the tray to press firmly against each other.