Originally Posted by DocP

I wouldn't have phrased the question that way, but you officially enter the US inside your Canadian departure airport when flying from Canada to the US. I once had a significant flight delay from Toronto, and spent several hours in the Transborder area. At that point I was legally in the US, even though I was physically in Toronto. If I had wanted to get food or shop airside, I would have had to exit the US and re-enter Canada. The flight then lands as a US domestic flight at the destination US airport, as all border formalities were done in Canada.