Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

The frequency of a wave is the reciprocal of the period of the wave.
The frequency of a wave is the reciprocal of what other attribute of the wave?

Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only contract.
Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?

An electron moves to a shell farther from the nucleus after it absorbs energy.

Where does an electron move to after it absorbs energy?