Instructions: In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Input: What makes duramen special?, Context: Heartwood (or duramen) is wood that as a result of a naturally occurring chemical transformation has become more resistant to decay. Heartwood formation occurs spontaneously (it is a genetically programmed process). Once heartwood formation is complete, the heartwood is dead. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once.
Output:
a naturally occurring chemical transformation