Yesterday, I bought my small son a Valentine's Day balloon. We picked a nice helium-filled mylar and I was very picky to get a full one because I wanted to get my money's worth (those balloons are expensive!). But when we took it outside, the balloon didn't look full at all.
At first, I was mad. "But I checked!" I thought. "I was careful to get a fully inflated balloon." When I thought about it more, I realized something. So I decided to keep the balloon and test my theory. And I was right! When I took the balloon into my house, lo and behold, it was full again.
What was going on? There was about a 50 degree F temperature difference between the store (roughly 75 degrees F) and the outdoors (25 degrees F). The molecules of helium gas in my balloon moved slower in the colder temperature. This decrease in molecular activity caused the volume of gas to decrease in the balloon. So, even though there was the same amount of helium in my balloon both times, the volume of space it took up changed because of the temperature difference.
You know, I've often heard that fact about gases, but it's just been something I took on faith. Seeing my balloon deflate -- and realizing why -- was fun!