http://home.howstuffworks.com/pop-up-timer.htmPOP UP TURKEY TIMERS
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These fun little contraptions always add to the lively atmosphere of Thanksgiving dinner and might be your only source of entertainment if Grandma still hasn't invested in a color tv. Though they appear to work off of magic, there's actually science behind the mystery.
These are made of a stick with a spring in a case held into place by a soft metal in the tip of the casing. The metal melts at about 185 degrees fahrenheit, which is the desired cooking temperature of a turkey. When the metal melts the potential energy from the spring turns into kinetic energy which pushes the spring up so you don't have to go through the annoying hassle of reading 185 F ...5/9 * (185 + 40) - 40 = 85 C off of a thermometer ever again!
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Fun fact that the pop up turkey timer manufacturers don't want you to know.... You can reuse them! Is all you have to do is heat the tip of the thermometer again by putting it into hot water (185 degrees F) which will melt the metal and you can then push the stick back down and allow it to cool into place.