When did our German get so good?

It is always a culture shock to arrive in Germany and have to switch into speaking and reading in another language. Usually when we come in the summer we spend the entire time with German-speaking friends and family. But this time, we set off for the UK after a week, with German burbling in our heads. When we landed in London and could communicate with abandon, I thought: "Wow, our German has really improved quickly." It was a strange kind of disconnect, like dreaming you're eating apple pie but it tastes like steak.