A puzzle from the excellent Riddler feature at FiveThirtyEight, via Oliver Roeder’s 2018 collection The Riddler:

Your eccentric friend Nigel flies from Heathrow to an airport somewhere in the 48 contiguous states, then hires a car and drives around the country, touching the Atlantic and Pacific Oceans and the Gulf of Mexico, then returns to the airport at which he started and flies home. If he crossed the Ohio River once, the Missouri River twice, the Mississippi River three times, and the Continental Divide four times, then there’s one state that we can say for certain that he visited on his trip. What is it?

	SelectClick for Answer
Minnesota. In order to cross a river an odd number of times and still return to the original airport, Nigel must “go around” the river somehow. That’s not hard with the Ohio, but the Mississippi is enormous and practically divides the country. So in order to cross it three times and return to his airport, at some point he’d have had to go around the source at Lake Itasca, Minnesota.