If I remember correctly, it’s not the freezing point. Fahrenheit used a brine that included ammonium chloride to set 0 on his scale since it was the closest thing he could make in his lab that was a consistent temperature. The other end was body temperature, which he set at 96 if I’m remembering right since it’s more easily divisible than 100. He was a little off on his body temperature measurements so it’s considered a little higher than that now.