Can you turn a cup of water upside down without any of it coming out?
X-Peri-Mas has already done a few experiments that use water to do a trick and this one is probably my favourite. Whenever I do this in front of a crows there is always a little gasp of “Wait, what?” when the trick works out. I also get a volunteer to come to the front and learn how to do it there and then in front of the audience so this is definitely one you can learn yourself!
What you will need
- A jar or cup
- A sieve or a reusable mesh bag (the kind you get with fruit in a supermarket)
- Elastic bands
How to set up
There are two ways to do this – one uses a jar, its lid and a sieve and the other uses a cup the first one is the way I learned to do it years and years ago and involves cutting through some metal so please do be careful – metal edges can be very sharp.
The jar and the sieve
Take a jar and poke a hole in the top of the lid. You’re aiming to make this hole the same size as the opening to the jar – so that all is left is the rim around the outside of the lid. You can use pliers to bend the metal edges and fold them underneath (so that the sharp bits point into the jar at the end) to keep your fingers safe.
Next cut a piece out of the sieve that is slightly bigger than the lid of the jar and place it so that it covers the hole that you’ve cut in the jar lid. Now screw the lid back onto the jar so that the opening to the jar is now covered with a sieve.
The cup and the mesh bag
There is a big push to remove plastic bags from our lives so that less plastic waste ends up in the ocean or in humoungous landfill sites. One of the ways I’ve seen to do that is to get rid of the tiny plastic bags supermarkets used to keep near fruit – you’d only use them once and then chuck them away (big waste). Now, you can get bags that are made of a fabric mesh next to the fruit aisles in these shops and they are perfect for this trick.
Place the mesh bag over the top of the cup and then use an elastic band to secure it in place.
You’re set – this one is easy.
How to do the trick
Fill your container with water.
What you need to do is get the container to be exactly upside down as smoothly as possible without spilling the water. We’re going to put a little lid onto the container to help get it upside down in the first place. Place a card or smooth piece of plastic on the top of the container – over the top of your mesh or sieve.
Holding the lid tightly against the jar, flip the whole thing upside down so that the bottom of the container is now pointing directly up and the mesh/sieve is pointing straight down at the floor. It’s important that you get this pointing exactly upright otherwise you’ll get wet shoes.
Take your hands off the lid – it should stay exactly where it is. This is part one of the trick because your audience will be surprised that the lid that isn’t really attached in any way is stuck to the container as if it was glued there. Ask your audience what would happen if you took the lid away…
…and then take the lid away. A few drips might fall but hopefully your water will stay trapped as if by magic, defying gravity in your container.
So what’s happening?
If you haven’t seen X-Peri-Mas #4 then now would be a good time to check that out. We talked all about pressure and how that can push air up into a container when you try and pour water out of it. The air going up gets in the way of the water trying to pour downwards and it’s all of this ‘getting in each other’s way’ that causes this trick to happen.
All the water in the jar is being pulled down by gravity. As it tries to move downwards it leaves a gap behind it – a totally empty space called a vacuum. Vacuums are really hard to make and they are even harder to keep empty – stuff loves to fly up and get into a vacuum (that’s why vacuum cleaners work!) so the air outside your container tries to get past the water to get to the vacuum that is forming. As the water tries to fall, the vacuum that is forming behind it sucks the water back up and defies gravity – the water is stuck where it is.
The air goes up, the water comes down. If there was a big opening then these two could pass by each other with no problems at all and all the water would pour straight out – but we have put a mesh in the way. That mesh has got loads of tiny holes in it which means that each tiny hole has a small bit of water and a small bit of air competing to get through – bashing and bumping into each other. There is then a stalemate and nothing moves anywhere!
If air could get past the water, the vacuum would be filled with air and then the water wouldn’t be sucked up any more, so it would fall!
Once you turn the container just a tiny bit you have ‘told’ the water and the air how to get past each other! The air wants to go upwards and the air wants to head straight down. When you tip the jar suddenly one side is higher than the other! The air will head towards the higher end and the water will head towards the lower end – now there isn’t the same bumping-into-each-other competition that there was before because one side has all the air and one side has all the water. The air can now get to that gap left behind as the water comes out and stops a vacuum from forming – the water keeps on pushing it’s way out.
Try this out at home – it’s really simple to do but always gets a WOW moment and remember to share your attempts with me using #XPeriMas on Twitter!