Given a scientific passage and an answer, generate a question for the given answer.

Passage: If water is so simple, why is it special? Water is one of the few substances that exists on Earth in all three states of matter. Water occurs as a gas, a liquid and a solid. You drink liquid water and use it to shower. You breathe gaseous water vapor in the air. You may go ice skating on a pond covered with solid water ice in the winter. Answer: three matter states
In how many states does water exist on earth?

Passage: Some nonmetals are very reactive, whereas others are not reactive at all. It depends on the number of electrons in their outer energy level. Answer: reactivity
The number of electrons in the outer energy level determines what property of nonmetals?

Passage: In eukaryotic cells, the start of transcription is one of the most complicated parts of gene regulation. There may be many regulatory proteins and regulatory elements involved. Regulation may also involve enhancers. Enhancers are distant regions of DNA that can loop back to interact with a gene's promoter. Answer: start of transcription
Which is one of the most complicated parts of gene regulation in eukaryotic cells?