Question

How do you explain water's ability to expand upon freezing?

Answer 1

Water froms a relatively open, honeycomb-like structure when it freezes, unlike most solids that pack into a close-packed or nearly close-packed arrangement.

Explanation:

Here is a picture of the water ice structure from
https://www.nyu.edu/pages/mathmol/textbook/info_water.html:

See https://socratic.org/questions/how-do-water-s-relative-densities-as-a-solid-and-a-liquid-differ-from-that-of-mo#264412 for a more detailed explanation.