Reviewing my flight history:
-CLE
-FLL
-IAD
-MDW
-MEM
-MIA
-MKE
-MSP
-ORD
-SFO
-SLC
(A lot of airports beginning with "M" on that list)

Of note, for counting states visited, I always find it interesting when people say they don't count states if they have just had a layover at an airport in that state. When it comes to MSP, I've connected there at least two dozen times. My total time in MSP probably adds up to close to 2 total days - more time than I've spent in many other states. SLC was in a similar status for a while but I drove through Utah moving from Washington to Texas.

Internationally I can put AMS**, LIS, MAD, SIN, SNN, and ZRH on this list.
**I did leave the airport during an extended layover at AMS and took the train to Central Amsterdam to explore for a couple hours but AMS was not my destination.

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