We use thermometers to check everything from the weather to a fever, but the familiar instrument we know today took more than a century to develop.

One of the earliest breakthroughs came around 1592, when Italian scientist Galileo Galilei developed an instrument called a thermoscope. It could show whether something was becoming hotter or colder—but there was one important thing missing: it didn't have a numbered scale, so it couldn't tell you the actual temperature.

During the 1600s, other scientists improved upon the idea by adding scales and experimenting with liquids inside sealed glass tubes.

Then, in 1714, German physicist Daniel Gabriel Fahrenheit developed a much more accurate thermometer using mercury inside a glass tube. Mercury expanded and contracted predictably as temperatures changed, allowing measurements to be made with much greater precision.

Fahrenheit later introduced the temperature scale that still carries his name.

A few decades later, Swedish astronomer Anders Celsius introduced another temperature scale. His system eventually became the familiar Celsius scale, in which water freezes at 0°C and boils at 100°C under standard atmospheric pressure.

So, who invented the thermometer?

There really isn't just one answer. Galileo helped start the idea, Fahrenheit created a highly practical and accurate thermometer, and Celsius helped give us the temperature scale used throughout most of the world today.

From a simple glass instrument in the 1500s to today's digital and infrared thermometers, it took generations of inventors to teach us exactly how hot—or cold—something really is.

Fun Fact:

The word "thermometer" comes from Greek words meaning "heat" and "measure."