I observed something interesting yesterday. At first, I couldn't explain it, but I figured it out after a few minutes, and I figured it might be interesting enough a problem to pose here. Yesterday, on my way to work (at about 7 in the morning) I picked up a pint of milk from UDF and drank it in the car (cause I gotta have strong bones). When I finished it, I put the cap back on and tossed it in the passenger seat. The next time I saw it was when I was cleaning my car out at about 10 at night, after a long, very hot day. Of course, it was night at this point, so it was a little cooler now than it had been in the middle of the day, but it was still significantly warmer than it had been that morning, so imagine my surprise when I saw that the milk bottle had contracted! How is this possible?