Gabriel Fahrenheit didn't actually discover the mercury thermometer. The concept of using a liquid's expansion and contraction with temperature to measure heat had been around for centuries. Sanctorius is credited with creating the first rudimentary thermometer in the early 1600s, using alcohol in a glass tube.

Here's what Fahrenheit did:

* He improved the design: Fahrenheit's main contribution was refining the design of the thermometer. He switched from alcohol to mercury, which was more sensitive to temperature changes and had a wider range of expansion. He also developed a more accurate and reproducible scale for measuring temperatures.

* He created a standardized scale: Fahrenheit established a temperature scale that was more precise and consistent than previous scales, allowing for more accurate temperature measurements. This scale is still used in some countries today.

In summary:

* Fahrenheit didn't invent the thermometer, but he made significant improvements to its design and created a standardized temperature scale. This is why he is often credited with the discovery of the mercury thermometer.