Angles of a Quadrilateral

Simple Question

a) What is the minimum number of acute angles in a convex quadrilateral? Prove.

That may be too easy. So, I'll give a point to (or split point between) the most creative and entertaining answer(s).

b) What is the minimum number of acute angles in a concave quadrilateral? Prove.

Clarification:

By acute, I mean angles that are 90 deg or lower. Acute = Non-obtuse.