Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: The Chemistry of Carbon What makes organic compounds ubiquitous is the chemistry of their carbon core. Recall that carbon atoms have four electrons in their valence shell, and that the octet rule dictates that atoms tend to react in such a way as to complete their valence shell with eight electrons. Carbon atoms do not complete their valence shells by donating or accepting four electrons. Instead, they readily share electrons via covalent bonds. Answer: carbon core
What makes organic compounds ubiquitous is the chemistry of their what?