It sounds like magic, but it is pure physics. The Eiffel Tower is made of wrought iron, and like all metals, iron expands when its temperature increases. On a hot summer day in Paris, when temperatures can reach 30°C or higher, the iron structure absorbs heat from the sun and its atoms vibrate more vigorously, pushing slightly farther apart. This thermal expansion causes the entire tower to grow taller—and also slightly wider.

Engineers have known about this effect for centuries and account for it in major construction projects. The expansion isn't random or dangerous; it follows a predictable pattern described by the coefficient of thermal expansion, a material property that tells us exactly how much a substance will grow for each degree of temperature change. For iron, this is a tiny fraction, but when you apply it to a structure 330 metres tall, those small fractions add up to the measurable 15-centimetre growth observed in the Eiffel Tower.

This is why bridges, railway tracks, and tall buildings include 'expansion joints'—gaps and flexible sections designed to absorb growth and contraction without cracking. Without these safety features, structures would develop stress fractures as materials expand in summer and contract in winter. When Gustave Eiffel designed his tower in 1889, he understood this principle well and built in the flexibility needed for Paris's temperature swings. So the next time you visit the Eiffel Tower on a scorching summer day, remember: you are standing on a structure that is literally taller than it was on a cool spring morning.