Episode 2: Waterproof Hankie
This surprising trick is a great excuse to threaten to pour water over your family. Put their trust to the test as you turn a full glass of water upside down over their head!
Before you start
These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.
What you’ll need
- A glass or transparent cup
- A plate, one with a raised edge works nicely
- Fabric handkerchief (or any type of cloth really). Don’t use a paper hankie, it won’t work!
What to do
In this activity you'll be tipping glasses of water upside down and there will probably be some spillages along the way. We recommend trying the trick outside, in a bathroom or at the very least over a bowl! You should definitely practise the motion a couple of times before trying it with a person.
- Push the centre of the hankie into the glass, so that the edges are hanging over the outside of the rim of the glass.
- Pour water into the glass, through the loose hankie. Make sure that the rest of your family can see the water pouring easily through the hankie into the glass. Keep pouring the water until the glass is roughly half full.
- Pull the corners of the hankie so that the material is taut over the top of the glass. Hold the glass and hankie so that the material stays tightly stretched over the opening.
- Place the plate on the top of the glass and tip it all upside down, being careful to keep the hankie pulled tight.
- Choose the member of your family most likely to forgive you if this goes wrong and you soak them…
- Hold the upside-down glass and plate combo above their head, making sure that the glass is vertical and the hankie is tight. Remove the plate and... voila! The water stays inside the glass.
What to talk about
- With no hankie, what force makes the water fall out of the glass?
- Have you ever seen a bug walk on water?
What’s going on?
There are three main forces that have an effect when we turn our glass of water upside down. Gravity pulls down on the water, and is the force that makes the water pour out of a glass.
There are also upward forces on the water due to both air pressure, which pushes on everything around us all the time (in this case the air will be pushing up through the mouth of the glass) and surface tension, which tries to hold the water together across the mouth of the glass.
Surface tension is what gives water an elastic-like skin at the surface. This elastic skin has real effects – it’s what pulls water into droplets like you might see on a spider’s web early in the morning, it’s what holds up water-strider bugs walking across ponds and it’s what lets you overfill a glass before it spills.
This trick makes use of the fact that the strength of surface tension depends on the size of the hole; the smaller the hole, the stronger the surface tension. Without a hankie, the hole is large and the surface tension is nowhere near strong enough keep the water together. The upward forces aren’t big enough to balance the downward force of gravity and so the water pours out (and as the water moves out of the way, air pressure makes the air rush into the glass to replace the water).
A hankie is made of a material that’s woven together and has tiny holes. Putting one across the mouth of the glass makes the hole through which water is trying to get through much smaller and so the surface tension much stronger. There is a strong stretchy skin across each hole. The surface tension, combined with the push due to air pressure is large enough to balance the force of gravity so that the water stays inside the glass – and you stay friends with your family.
If you want to investigate further with your family, you can put the strength of surface tension to the test (do this bit over a sink, not over somebody’s head!)
- Almost fill a glass with water and put Clingfilm over the mouth. Turn it upside down like you did with the hankie. Of course nothing happens… Clingfilm is waterproof!
- Can your family predict what will happen if you keep it upside down and prick small holes in the Clingfilm with a pin?
- How many small holes can they make without the water escaping?
- How big can you make the hole before water starts to drip through?
Now that you’ve found out about surface tension, gravity and air pressure, can you work out what’s going on?
Did you know?
The surface tension of water is pretty strong, but it’s the element mercury that has the highest surface tension.
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