My parents and sister are visiting us. Last night, we were downstairs when my sister asked: why is it so cold downstairs, and not upstairs? I replied with: because heat goes up.
Today, it struck me that it's been pretty warm at night (upstairs). And then I thought of a better explanation for the heat differential: a couple of weeks ago, when it was pretty hot outside, I had the idea of closing all the vents downstairs, so that the A/C would come out just upstairs, where it was hotter. But the weather changed and it's been pretty chilly these days, so now the heat is just coming out upstairs, while downstairs it's freezing. Ooops.