How does water instantly freeze in a water bottle?

In our garage we have some bottled water. With the colder weather I’ve
witnessed an interesting phenomenon. When I bring a water bottle into
the house, it is entirely liquid, but as soon as I shake it, it fills
with slush.

Apparently, it’s a process called
supercooling.
The water temperature is brought below the freezing point, and all it
takes is a little shake to freeze the water. Pretty cool!