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As kids, we suspected it was Dracula's blood. As adults, we learned the truth. Sadly, it's a lot less interesting. In most cases, the red stuff you see in a thermometer is either alcohol or mercury. The California Energy Commission offers an easy-to-read explanation of how thermometers work. Simply put, alcohol and mercury expand when heated. In a standard bulb thermometer, this means the liquid will rise as the temperature increases. Howstuffworks features a section on how to make your own thermometer out of household stuff (jar, straw, chewing gum, etc.). Of course, a store-bought thermometer will run
you only about $5, so this may not be the best use of your time, but our craft-oriented readers may find it interesting. Many people believe good ol' Galileo Galilei was the inventor of the thermometer, but his invention was actually more of a "thermoscope." It indicated changes in temperature but didn't measure them. Santorio Santorio took the thermoscope to the next level when he added a scale. Whether or not he used Dracula's blood in his prototype is unknown.
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