The Fahrenheit temperature scale is based on the temperatures that Mr Fahrenheit considered to be significant.
He thought that if you put enough salt into ice you could get it down to "absolute zero", so he put a lot of salt into a a large volume of ice and he measured the coldest temperature he could get. He called that "zero".
He thought that the next most important temperature would be the temperature of the human body. He measured HIS temperature and called that 100 degrees. Of course, his temperature was a little high, so he probably had a slight fever.
It's interesting that the Fahrenheit scale was based on flawed science, and we still use that scale today. When will the U.S. convert to metric?