Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.
Example: Who was the first American President?
Example solution: president of the united states
Example explanation: The question talks about the president of the USA.

Problem: What SI unit (International System of Units) of measurement is defined as The amount of substance that contains an equal number of elementary entities as there are atoms in 0.012kg of the isotope carbon-12?
Solution: amount of substance