Look closely at the Eiffel Tower. Not at the shape, at the joints.

Nothing up there was welded. 18,038 iron parts, 5,300 drawings, and about 2.5 million rivets, driven hot by teams of four.

The clever part: hot iron is slightly longer than cold iron, so as each rivet cooled it shrank and pulled the plates together with a grip no one could tighten by hand.

So why did we stop building like this? Slow, dangerous, and every joint depended on four people and one hot rivet. By the 1960s, high-strength bolts had largely taken over.

Next time: what the numbers stamped on a bolt's head mean.

Save this for later. Sources: official Eiffel Tower history page (toureiffel.paris).