Stick two glasses together
Hot water is poured into two rigid glasses. Once warm the glasses are placed with their open ends together and a piece of damp paper towel in between. Once cool the glasses are stuck together and an audible pop is heard when they are separated.
- Two rigid plastic (not glass) glasses.
- Hot water
- Damp kitchen towel
Pour hot water (as hot as possible) into the two glasses. Allow both glasses to warm up and tip out the water again. Put the two glasses together, mouth to mouth, with a piece of damp kitchen towel between them. When the glasses cool down they can both be picked up by the upper glass. There will be a distinct sound of air rushing in when the glasses are separated.
The hot material in the glasses cools and contracts. The pressure inside the glasses is then less than atmospheric pressure. It is thus the atmospheric pressure that is keeping the two glasses together when they are picked up but the upper glass. The fact that the glasses have been heated by water helps the effect. The gaseous water cools and condenses to liquid water which makes the pressure differential much greater than if it had only been done with hot air.
Many plastic glasses have a rounded rim which makes sealing the glasses to one another difficult. It helps if the rims can be flattened and this can be done using a fine sandpaper.
This can be difficult to get to work. It is sometimes worth having a second set ready to try if the first one is unsuccessful.
This demonstration involves manipulation of very hot water – suitable precautions need to be taken. The glasses will remain hot for some time after the water has been tipped out. Do not use glasses made of glass as the stresses of the hot water being poured in and the pressure differential on cooling may cause them to break.