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Using scissors, just snip the end of the balloon right off. There’s no precise place to make the cut. You just want to make sure that the opening will be large enough to fit over the mouth of your jar. As pressure rises, the pointer will aim higher up. When pressure falls, the pointer will, too. Watch the magic happen, and make marks when you notice that the pointer has changed position.  If you like, you can label the pointer’s initial position “1” and then number each new position in order. This is a great idea if you want to use the barometer for a science project. The barometer works because high air pressure pushes down on the balloon, causing the pointer to move upwards, and vice versa. Make notes about the weather conditions associated with each change in the barometer's position. When the pointer rises in times of high pressure, is the weather cloudy or clear? What about when the pointer drops during low pressure? Low pressure is usually associated with rainy weather. High pressure can mean mild or cooler weather.
Cut the neck off of a balloon. Record changes in the pointer’s position. Interpret the results.