Sure, negatives aren’t hard, nor are decimals. But I should remind you we’re talking about a population that wouldn’t buy a third-pounder hamburger because they thought a quarter-pounder was more. Fractions are covered pretty early on, too!

Joking aside, if F actually was based on something specific and measurable, it’d also make sense. Then it’s just a matter of what you got used to. Granted, human temps vary, so you can’t just make 100 the human temp and 0 the temp a human dies, so that’s an impossibity. (Water can vary too under circumstances if I remember right, but not quite as much or as unpredictable as some human based metric).