Important question

If you change planes in a country but don't leave the airport, does it count as entering the country? What if you're entering/leaving Schengen there so you go past passport control, does that make a difference?

I'm trying to work out which countries in our pinboard of where we've been to stick pins in, and while on the way out we stopped in Denmark, which we'd already been to, only I had previously been to Sweden.