system
Given the following post, respond as if you're commenting on Reddit:
human
Why do most substances in the liquid state thicken as they cool down towards a solid, but some substances, such as water, suddenly become solid at freezing point rather than thickening in a gradient as it cools to freezing point?
gpt
The viscosity of water does change with temperature, as do all pure liquids.  Going from boiling down to the freezing point, the viscosity of water more than triples.  The other liquids you are thinking of are probably on their way to a glass transition rather than freezing into a crystalline solid.  In this case, the material will appear to get more and more viscous until it ceases to flow altogether.

Many mixtures exhibit the behavior you are describing, though.  For example, mixtures of alcohols and water get very viscous when they are cooled significantly below 0C.