The Eiffel Tower in Paris, France is mostly made out of iron beams. When a substance is heated up, the atoms and molecules inside of it move apart from each other. This is because the atoms and molecules gain more energy as they heat up. As they move apart, the material gets longer or bigger! This is true with iron, which is what the Eiffel Tower is made out of. The summer heat causes the iron to expand. That is why the Eiffel Tower can be 15 cm taller in the summer!