The difference between 0°C and 0°F

Okay, I know this is a really simple thing, but it's something that I've never really given much thought to before. So we all know that the freezing point of water is 0°C while it's 32°F, and the boiling point of water is 100°C while it's 212°F. But what I've never really given much thought to is the actual difference between 0°C and 0°F. <br><br>Now, I know that 0°C is 32°F and that 0°F is -17.8°C, but my question is, *why?* Why did Celsius and Fahrenheit set 0°C to 32°F and 0°F to -17.8°C?