system
Given the following post, respond as if you're commenting on Reddit:
human
Why do most liquids have a freezing point instead of just gradually getting more viscose?
gpt
this is because the transition from liquid to solid is accompanied by a structural change from amorphous to crystalline. There are some conditions under which materials which will "freeze" in an amorphous "glassy" state, referred to as vitrification, but this usually happens much more quickly than regular freezing, or requires diluting the liquid with 'antifreeze' agents which disrupt crystal formation. 

The difference in the amount of vibration energy that the substance's molecules have in its free-flowing liquid state, vs its regular, confined, crystalline state is called the heat (enthalpy) of fusion (fusion = freezing). This heat is what causes the freezing point plateau in which the phase changes from liquid to solid, as the temperature stays the same. 

Source: BS in chemical engineering.