What is the same in Celsius and Fahrenheit?

The story behind the answer

Fahrenheit (F) and Celsius (C) relate by F = (9/5)C + 32. To find when they’re equal, set F = C: C = (9/5)C + 32 → 5C = 9C + 160 → -4C = 160 → C = -40. So -40° is the unique temperature where both scales give the same reading.

Context: Celsius sets water’s freezing at 0° and boiling at 100°, while Fahrenheit uses 32° and 212°, so they have different zero points and step sizes. Their lines intersect at -40°. Memory tip: “It’s so cold, the scales agree—minus forty.”