It sounds like magic, but it's pure physics. The Eiffel Tower is made of wrought iron, and like all metals, iron expands when its temperature rises. On a scorching Paris summer day, the metal framework absorbs heat from the sun, and the atoms vibrate more energetically, pushing slightly further apart. This thermal expansion causes the entire structure to grow taller—and wider. Engineers have carefully measured this effect for over a century, and the tower can swing by up to 15 centimetres in height between winter and summer.

This isn't unique to the Eiffel Tower. Any large metal structure experiences thermal expansion. Bridges, skyscrapers, railway tracks, and even airplane fuselages all grow and shrink with temperature changes. Engineers must account for this when designing: they install special expansion joints that allow materials to move freely without cracking or buckling. Without these joints, a bridge might buckle in summer heat or develop dangerous gaps in winter cold.

The Eiffel Tower's expansion also causes it to tilt slightly away from the sun as the metal on the sun-facing side heats up faster than the shaded side. On an extremely hot day, the top of the tower can shift by up to 15 centimetres horizontally. This is why engineers constantly monitor the tower's geometry—not out of concern, but out of precision. The structure has been standing for over 130 years precisely because its designers understood and respected the physics of materials.