Question

The telephone wires which sag in summer become straight in winter. Why does this happen

Answer 1

In winters, the wires contract due the cold climate.
In summers, the wires sag due to the hot climate.

Answer 2

 During winter the wire's temperature drops and thus it contracts in length and becomes straight thus the wire becomes straight in winter. It happens due to the contraction of the molecules.

During summer season the temperature of the wire rises and it thus expands in length and thus it sags down. This occurs due to the expansion of the molecules.