To make your barometer, use the clear glass bottle and the clear glass jar. Place the bottle upside down into the jar, without touching the bottom of the jar (peanut butter jars work really well for this project, if you can find a brand still available in glass jars). Fill the jar with water so that it comes up an inch or two over the mouth of the upside-down bottle. Add some food coloring to the water, and angle the jar and bottle just enough to let some air escape.
Slide the rubber band around the jar -- this will be your marker line -- at the water line. Place the barometer in a spot outside, but not in direct sunlight. As the water level rises and falls due to barometric pressure, mark the new level with a Sharpie marker or additional rubber bands. As pressure in the air increases, the water in the jar gets pressed down, which then makes the water go into the open bottle. As pressure drops, air will rise and eventually cool -- in your barometer, the water will drop away.
