What is the freezing point of water in Celsius?

Zero, by construction - the scale was defined around it, and Anders Celsius originally had it the other way up, with zero for boiling and 100 for freezing. It was reversed shortly after his death in 1744.

Water does not reliably freeze there, though. Freezing needs a nucleation site to start on - a speck of dust, a scratch in the container, an existing ice crystal - and very pure water in a very smooth vessel can be cooled to around minus 40 before it gives in. Clouds are full of supercooled droplets waiting for a surface, which is why aircraft ice up: the wing provides one.

The definition has since moved out from under the whole thing. The kelvin is now defined by fixing the Boltzmann constant, a number relating temperature to energy, so the modern scale does not reference water at all - and as a consequence water's freezing point is no longer exactly zero but a measured quantity that lands a hair away from it. The schoolroom fact and the formal definition have quietly parted company.