On November 7, 1940, four months after it opened, the first Tacoma Narrows Bridge twisted itself apart in the wind. This Saturday is the anniversary, and it makes one of the best 30-minute lessons in cause-and-effect thinking you can run with a class, a club or your own family.
The reason is that almost everyone already "knows" the answer, and the popular answer is shaky.
The tidy story most people remember
The familiar version goes like this: gusts pushed the bridge at exactly its natural frequency, like someone pushing a child on a swing, and the swaying grew until the deck broke. It sounds right and it fits the famous film.
The trouble is that it treats the wind as a steady metronome tapping the bridge from outside. Ask a group "how would the wind know the bridge's rhythm?" and the story starts to wobble.
A 30-minute investigation plan
1. Separate what was seen from what is claimed (5 minutes). Have everyone write two lists. Observations: the deck was unusually slender and flexible, it moved up and down for months (locals called it Galloping Gertie), and on the last day the motion changed from up-and-down to twisting. Claims: "it was resonance", "the wind matched its frequency". Most groups find their lists are mostly claims.
2. Build a short timeline (5 minutes). July 1940 the bridge opens. Through summer and autumn engineers study the motion and try fixes. November 7 the vertical motion turns into large twisting and the main span falls. March 1941 the federal investigation points to excessive flexibility and the need for aerodynamic research. October 1950 a stiffer replacement opens.
3. Offer the alternative mechanism (10 minutes). Instead of the wind pushing on its own schedule, consider a loop: the twisting deck changes how air flows around it, and that changed airflow feeds more energy back into the twist. The bridge and the wind are now one system. Ask: what would we expect to see if this were true, and what if the simple swing story were true? Which better explains why the motion changed on the final day?
4. Name the causes at different levels (10 minutes). The immediate cause (the twisting motion), the design choices behind it (a very slender, flexible deck), and the knowledge gap (bridge designers of the time had little aerodynamic testing to lean on). Good investigations rarely end with one cause.
Where to take it next
Faultory has this as a free, guided case, Tacoma Narrows, 1940: follow the energy, not just the wind, with the timeline, photos and evidence to sort, plus other cases like the 2003 Northeast blackout.
If you want to run this kind of session on paper, the Investigator's Field Kit is a paid set of 13 printable worksheets for working out why something happened. Disclosure: I'm connected to both, and this post was written with AI assistance.