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

[Q]: Passage: The two materials do have at least one thing in common. The atoms in the materials are held together by covalent bonds. These bonds consist of electrons shared between two or more atoms. Unlike ionic bonds, where electrons are either lost or gained by an atom to form charged ions, electrons in covalent compounds are shared between the two atoms, giving rise to properties that are quite different from those seen in ionic materials. Answer: ionic bonds
[A]: Electrons in covalent compounds are shared between the two atoms, unlike the case in what type of bonds?


[Q]: 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
[A]: In how many states does water exist on earth?


[Q]: Passage: Models may be physical, conceptual, or mathematical. Answer: mathematical
[A]:
Models may be physical, conceptual, or what else?


