For the same reason the liquid in a thermometer rises and power lines droop on a hot afternoon. The scientific term is thermal expansion -- as molecules warm up they tend to take up more space, sort of like how you might want to give a boxer some room as he warms up.
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For the same reason the liquid in a thermometer rises and power lines droop on a hot afternoon. The scientific term is thermal expansion -- as molecules warm up they tend to take up more space, sort of like how you might want to give a boxer some room as he warms up.