Saturday 11 January 2014

The magic upside-down glass of water!

Do you want to know how to turn a glass of water upside down without the water falling out? 
Well here is how to do it and of course, you know all ready I am sure, it's not really magic, it's SCIENCE!  (You don't need to tell your friends that though!)

So here's how to do your magic trick...
You will need
  • A glass
  • An Index card, postcard or something similar made of thin cardboard.
  • A sink or a bowl if you're not doing this one outside.


Fill your glass with water.


Put the index card over the open end of the glass making sure that all the edge of the glass is touching the index card.
Turn the glass with the card still pushed against it upside-down like this... 

If you do it right the index card will stick to your glass and the water will remain in the glass.  Not only that but your friends will be amazed.

How does it work?

Two reasons really.  It works because of the air pressure all around up pushing on the index card, so why doesn't the weight of the water force the index card off? Well you see the gap in the glass just above the water. That air, because there wasn't much space to go is a pocket of low air pressure that we have created causing a small vacuum and drawing the water towards it much like in the rising water experiment and stopping the water from pressing down hard enough to push past the index card.

The other reason is surface tension and adhesion.  Surface tension is created because of the way the molecules position themselves.  Oxygen molecules steak electrons from hydrogen molecules in a water molecule H2O, because of this water molecules become a dipole.

Dipole is a pair of equal and oppositely charged or magnetised particles separated by distance.  If you think of every water molecule as a tiny magnet where opposite poles attract because there is a small attractive force between each water molecule just like that and this is the force responsible for surface tension; the water molecules stick together to create an elastic like surface layer.

So adhesion?
Adhesion happens because the water molecules have these positive and negative ends that I have mentioned.  These are not just attracted to each other, they are also attracted to other materials. In this experiment the water molecules are attracted to the paper, adhering to it, while continuing to keep the surface tension with the other water molecules. This also helps to keep the index card in place.
So, to makes it as simple as possible, the index card doesn’t fall down because of the difference in pressure and it doesn’t slide off because of surface tension and adhesion. So, although it makes for a great magic trick it is really all about science.
 Other things to try...
  •  Use different materials to replace the index card, fabric, plastic etc.
  • Try putting a thin layer of soap round the rim, does it make a difference?  Be sure to do all these experiments over a sink!


  1. wow, I love reading your science posts, they give me lots of ideas to try.xx

    1. I'm pleased you find them inspirational! How lovely to hear. I have quite a few now as we just love practical science! :D

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