Prefix of the day:
From Latin "acus" meaning needle. We use ACU- most frequently to mean "sharp"--like a needle.
ACUte: sharp or intense--like ACUte pain or ACUte angles: ACUte angles are sharp angles (as contrasted with obtuse angles, which are dull)
ACUity: sharpness--either of senses or objects.
ACUtance: a measure of the sharpness (clarity) of a photograph.
ACUmen: keen insight or discernment; sharpness of mind.
A porcupine has a sharp bACU know (back, you know).