instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only contract.
answer:
Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?


question:
Ionic compounds contain cations and anions rather than discrete neutral molecules.
answer:
Ionic compounds contain cations and anions rather than what?


question:
Mercury is always closer to the sun than it is to earth.
answer:

Which of the following planets is always closer to the Sun than it is to Earth?