But look what happens when you knock the index card out of the way this time...
WHAT??? The water's still in the jar??? HOW???
The secret:
A piece of screen (regular window screen, found at your hardware store, or from a broken screen you have at home) placed inside the jar band.
Air pressure held the index card in placed, as discussed previously. Once the card is gone, surface tension takes over. The water molecules join together to form a membrane, strong enough to hold the water in, in the screen openings.
If you tip the jar, you'll break the surface tension, and...
This demonstration leads to some great experiments...
-how large of a screen opening can you use before the openings are too large to hold the water
-will other materials (cloth, etc.) work in the same manner
-what if you use soapy water
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PS While you're carrying out the demonstration, don't let the students see the top of the jar and the screen.
PPS You might want to do this with two jars and two volunteers. Set it up so that everything looks the same, except that one of the jars has a piece of screen in it and the other does not. Knock the index cards out of the way at the same time and SURPRISE! One of the jars spills all of its water, the other does not. Now, get someone to explain what happened!
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