cheap thermometers

This may sound silly, I don't know. I thought I should let everyone know something that happened with Charlie's thermometer that was bought from wal-mart. It was a cheap glass one with the tiny metal balls in the bottom. I was making my rounds and checking on my fish like I do and when I got to charlie's tank I noticed something weird with his thermometer. It was full of water. First thing that panicked me was the mercury leaking into his water, then I though about the metal balls... I freaked out and took it out of the tank in a matter of seconds after these thoughts. I checked it and to my eyes I couldn't see a single crack. Charlie was fine but I went ahead and did a total water change doused everything with hot water. Just wanted to tell everyone to keep an eye out if you have those types of thermometers.