Answer: Frozen Salt Water

If you’ve ever stopped to ponder why exactly the freezing point on the Fahrenheit scale is set at 32 degrees–a rather arbitrary number, all things considered–you’ll need to ponder all the way back to the 18th century.

It was then, in the 1720s, that Daniel Gabriel Fahrenheit was ironing out zero point for his temperature scale. The coldest temperature that Fahrenheit could produce in his laboratory was the result of mixing ice, water, and ammonia chloride to make a frigid slurry. This measurement became the zero-point for the scale. The second point of reference was the temperature of water when chilled to the point that ice began to form–pure water forming ice was assigned to 32 degrees. Fahrenheit later assigned 212 degrees as the value for boiling water because the 180 degree difference between the two is 180 degrees because 180 can be easily and evenly divided by a variety of numbers.

After a long period of metrification starting in the 1960s, the only countries that still use Fahrenheit for conventional measurements are the United States, Belize, and the British territory of the Cayman Islands.