A great excuse to threaten to pour water over your audience – but with a surprise twist thanks to physics.
• large glass
• ashtray or similar
1. Push the centre of the handkerchief into the glass, so that the edges are hanging over the outside of the rim of the glass.
2. Pour water into the glass, through the loose handkerchief. Make sure that your audience can see the water easily passing through the handkerchief into the glass. Keep pouring the water until the glass is roughly half full.
3. Pull the corners of the handkerchief so that the material is taut over the top of the glass. Hold the glass and handkerchief so that the material stays tightly stretched over the opening. For younger audiences you may like to say some 'magic words' that make the hanky water proof.
4. Place the ashtray on the top of the glass and tip it all upside down, being careful to keep the handkerchief pulled tight.
5. Choose a likely suspect from your audience to threaten with a drenching! Hold the upside-down glass and ashtray above their head, making sure that the glass is vertical and the handkerchief is tight. Remove the ashtray and voila! – nothing happens! The water stays inside the glass.
How does it work?
This demonstration is based on surface tension and air pressure. When the handkerchief is loose, the water can pour through the gaps in the fabric. However, when the handkerchief is pulled tight, the water molecules can form a single surface or membrane across the handkerchief due to a property called surface tension. The air surrounding us is exerting a force in all directions - air pressure. When the membrane is formed on the surface of the handkerchief, it provides a uniform surface for the air pressure to act upon. The force of the surrounding air acting upon this membrane is sufficient to overcome gravity, and so the water stays in the glass.
Tips for Success
Don't try to substitute a paper tissue for the handkerchief as it won't work! If the glass isn't held vertically, some water may dribble out where the membrane attaches to the edge of the glass.
This trick will work in almost any environment and with any age group.
Did You Know?
Galileo was among the earliest to demonstrate the existence of surface tension on water by showing that an iron needle can be floated lengthways on water, but not on it’s point.