As soon as the science magician places an index card over the top of a glass full of water, everyone in the audience is in awe: The water does not fall! To make this happen, the card transfers the force of air pressure upward to the water. The water and the card are attracted to each other through a property called adhesion, which also acts as glue. If the cup were to leak even a tiny bit, the air molecules would rush in and equalize the internal pressure, which would cause both the water and the card to fall.
The trick is to get the index card as close as possible to the water without letting it touch it, forming an airtight seal. This is easy enough to do, and the glass of water will remain suspended for a few seconds. After the water has cooled, it will begin to fall due to gravity.
This hands-on experiment is a fun way to teach students about the interaction of forces, such as gravity and air pressure. It also highlights the importance of invisible forces like friction and adhesion.
While it is unlikely that you will have a supply of paper cups when the SHTF, this trick could come in handy. It is one of the easiest ways to boil water without a pot. For a more advanced version, use a larger glass and try placing the index card at the bottom of the glass instead of the mouth of the cup.