What is the only u. s. airport that flies directly to all 6 major continents?

Chicago

Explanation:

Chicago is the only airport in the United States, thus making the city of Chicago as well, that has direct flights to six continents. This makes this airport the best connected in the United States, offering direct flights to North America, South America, Europe, Asia, Africa, and Australia & Oceania. The only continent left to which there's no direct flight is Antarctica, with the reasons being the small interest, but much more the safety concerns as Antarctica is not the best place to have flights to considering its weather conditions. Chicago is actually only one of five cities in the world that has an airport that offers direct flights to six continents, being in the company of London, Dubai, Johannesburg, and Doha.

associative property of addition

they use tree rings to figure out weather patters.