Water

The humidity here is incredibly wet. I have the air conditioner on "low" all day. I can hardly ever actually see out my window because of the condensation. Or the lack of light since it's night. But the water I pay most attention to is in my two glass water bottles.

Snapple bottles leak when you refill it with water. Or so I thought. When I put hot water into it (~80C), the Snapple bottle not only doesn't leak but has a slight vacuum seal. I thought about this for a while, probably too long, and realized that the quantity of air in the bottle is inversely proportional to temperature. When I put hot water in, there is little air; then I put in cold water, there is much air. As time goes on, the air in the bottle equilibrates with the temperature of the room. If there was too much air, there would be positive pressure, air (and water) leaking out easily. If there was too little, there would be negative pressure, a vacuum seal preventing any leaking.

Good ol' PV = nRT

=p