Why does water at certain temperatures contract when it becomes warmer?

Answer 1

Good question. No one really knows the answer. Note the "may" in what Wikipedia has to say on the subject:
http://en.wikipedia.org/wiki/Negative_thermal_expansion

And how this site describes it in detail, but avoids giving the reasons for it:
http://www.lsbu.ac.uk/water/explan2.html

You'll find phrases such as "the structure collapsing", but that seems like hand-waving to me.

But water isn't the only substance with this property. Wikipedia mentions cubic zirconium tungstate, which has a negative thermal expansion over a very wide range.

Copper and silver oxides also show this property:
http://www.esrf.eu/UsersAndScience/Publications/Highlights/2006/XAMS/xams04