Science Fair Project on Water Floating in a Jar From an Index Card

Setting Up

• Take a pint size jar and remove the twist-on lid. Position the plastic screen over the top of the jar. Screw the lid back onto the jar and then wait a few seconds before removing it. This process will create a circular indentation in the plastic screen for you to cut around. After cutting out the shape, fit the circular piece of screen on the top of the jar and put the lid on. Place an index card and a jug filled with water next to the jar on the area where you will be demonstrating the project.

The Project

• When it is time to demonstrate your project, unscrew the lid from the jar and pour in the water from the jug until the water almost hits the brim. Hold an index card over the top of the jar then turn the jar upside down. Remove your hand holding the index card and the card will remain stuck to the jar, despite all the water weighing down it and, of course, gravity. Carefully slide the card away from the jar and dumbfound your audience as the water stays in the upside down glass. Replace the index card and flip the jar back the right way up. Conclude your demonstration by simply pouring the water back into the jug by tilting the jar on an angle.

Showmanship

• While this project is already likely to attract the attention of anyone in the vicinity, there is no harm in adding some flair to the proceedings. Ask one of your friends to be a helper or you could even get someone from the audience to assist. When you remove the paper from the glass, do so over the volunteer's head. If you have practiced this trick enough, then you will know that the water won't spill, but the gasps it generates are priceless.

Explanation

• Now that your project has made an impact on everyone, it's time to explain how it works. The air pressure from the atmosphere pushes upwards and sideways, not just downwards. The index card stays attached to the jar because the air pressure pushing up onto the card is stronger than the weight of the water. Once the card is removed surface tension holds the water in place. The molecules in the water are joined through the force of cohesion, making the surface of the water create a kind of membrane between the openings in the screen. If you tip the jar then the seal breaks; this is why you can pour the water back out at the end.