If we're really getting into the temperature discussion: There's also the scientific unit of Kelvin where 0°K is the absolute coldest it can get, starting with the coldest point at 0 on a scale seems quite legit, we can all agree on that. A change of 1°K is exactly the same as a change of 1°C, so it's really easy to convert, just subtract 273 and you've got your temperature in °C. Compare that to multiplying by 9/5 and subtracting 460 or something like that.
Now using this scale day to day is not that reasonable, I guess we can agree on that too, so we need to use another scale that makes more sense for us as humans and define other points of temperature. At 0°C water freezes and at 100°C it boils, simple as that. We have water and this is easy to replicate. Fahrenheit was based on ... the coldest temperature in Danzig(?) in a particular winter being 0°F and the body temperature of a horse (or was it a human?) being 100°F. Well guess what, Danzig might not be 0°F this year (thanks global warming! ) and whatever animal it was might have a fever today. Congratulations!
Now that's like me saying I'll make my own scale and set 0°S (S for sheep ) as the temperature of my tap water and 100°S will be the temperature of my cup of tea that I will leave standing around for 37.8 minutes this sunday afternoon. You will notice, that the hot day sounds even hotter now, so let's all use that instead, right? I'm honestly just happy nobody has decided it was a good idea to add the unit "Fliegenheit" where 1 Fliegenheit is exactly 7 Fahrenheit.
OK, sorry, rant over.
@SteveYork: you're probably right, I'll edit that