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

Example Input: Passage: the fact that stars are made up of very lightweight hydrogen gas. Answer: hydrogen
Example Output: What type of gas are stars made up of?

Example Input: Passage: The electron cloud of a helium atom contains two electrons, which can normally be expected to be equally distributed spatially around the nucleus. However, at any given moment the electron distribution may be uneven, resulting in an instantaneous dipole. This weak and temporary dipole subsequently influences neighboring helium atoms through electrostatic attraction and repulsion. It induces a dipole on nearby helium atoms (see Figure below ). Answer: an instantaneous dipole
Example Output: A weak and temporary dipole that influences nearby atoms through electrostatic attraction and repulsion is known as what?

Example Input: Passage: When two or more pure substances are combined together, a mixture is formed. Unlike pure substances, mixtures have a variable composition. Variable composition indicates that the relative proportions of the mixtures components may vary, and they can be separated by physical methods. There are two main types of mixtures. Answer: variable composition
Example Output:
Unlike pure substances, mixtures have a what?