Take one out and scratch my head, I am now black but once was red.

This is a classic riddle with wordplay. A matchstick has a red tip (red head) and a wooden body. When you 'take one out' (remove it from the box) and 'scratch my head' (strike it against the box), the red tip burns and turns black. The phrase 'once was red' refers to the red tip before being struck. This type of question tests lateral thinking and the ability to interpret metaphors literally rather than looking for factual or scientific answers.