Ingredients
• large glass
• plate or similar
• water
• handkerchief (NOT paper – must be fabric)
Instructions
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 waterproof.
4. Place the plate 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 plate above their head, making sure that the glass is vertical and the handkerchief is tight. Remove the plate 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. |
