The Eiffel Tower looks solid and immovable, but its 7,300 tons of iron are constantly responding to the weather.

Iron expands when it gets warmer and contracts when it cools, so on hot days the tower actually grows slightly taller. When temperatures drop, it shrinks again. The changes are tiny, but on a structure more than 1,000 feet tall, they're measurable.

And getting taller isn't the strangest thing the Eiffel Tower does.

Sunlight and wind affect the structure in ways Gustave Eiffel and his engineers had to understand more than 130 years ago — and the tower has been quietly moving ever since.