Materials needed: .5 liter bottle, small drinking glass to fit snug against side of pop bottle, and electrical tape to seal. The bottle can't rest on the bottom of the drinking glass-there has to be a little space of the water to get into the pop bottle.

Instructions: Fill glass with enough colored water to cover the spout of the pop bottle. You can use your favorite color of food coloring. Take pop bottle and insert into glass and tape the edge of the glass to the bottle as shown. No air should escape. The seal should be air-tight. Set your new barometer somewhere where you can easily see it but don't disturb it. It may take a few days for the air inside the bottle to "even out". As the air pressure changes, you will see the water rise or fall accordingly. It goes up the spout when the weather is nice and it goes down the spout when the weather is bad.