TITLE: Are fields flat $\mathbb Z$-modules?
QUESTION [10 upvotes]: This is probably one of those questions with a super obvious counterexample, but here goes.
Is a field necessarily a flat $\mathbb Z$-module?

REPLY [18 votes]: Over a PID (such as $\mathbb{Z}$) being flat is equivalent to being torsion-free. Therefore, if your field is torsion-free, it is flat, and if it has torsion, it is not flat.

REPLY [12 votes]: A $\mathbb{Z}$-module is flat if and only if it is torsion free, so it might depend on the characteristic of the field.