A scale built around water
Why Does Water Freeze at Zero?
Celsius put zero and one hundred at water's freezing and boiling points, then split the gap.
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Step 1 of 5
A scale needs two points everyone can reproduce.
To agree on temperature you need two events that always happen at the same warmth, anywhere in the world, so any thermometer can be checked.
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Step 2 of 5
Water gave two convenient fixed points.
Water freezes and boils at dependable temperatures, and both are easy to reproduce in any kitchen, which made them an obvious choice.
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Step 3 of 5
The gap was simply divided into a hundred.
One degree Celsius is a hundredth of the distance between those two events. The number 100 is a convenience, not a discovery.
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Step 4 of 5
Nothing stops the numbers going past the ends.
The scale carries on below zero and above a hundred, exactly as a ruler continues past its markings. Minus twenty is a real temperature.
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Step 5 of 5
Other scales chose different anchor points.
Fahrenheit anchored on a salt-and-ice mixture and body heat, which is why its numbers look arbitrary and water freezes at 32.
There is one genuine bottom to temperature: minus 273.15°C, where atoms have no heat energy left to give up.
The short version
A scale needs two events anyone can reproduce anywhere, and the hundred between water's two is a convenience, not a discovery. Fahrenheit anchored on a salt-and-ice mixture and body heat instead, which is why water freezes at 32 on it.
Try it yourself
Put a thermometer in a glass of ice and water and stir. It settles at zero and stays there while any ice remains.