sometime in 1963 - I don't remember any more accurately than that .
I'd been reading about how hot air rises and the flow of air from the bottom of a door is from the cold room toward the warm room. The book I'd been reading gave a simple test; hold a lighted match to the top of the door and notice how the air flows from the warm room toward the cold; then repeat the experiment at the bottom of the door and see how it flows in the opposite direction.
So I tried it. Top of the door; yes, it flowed from the lounge room in which my parents were watching TV toward the somewhat colder room I was in. Bottom of the door. Yes, it flowed the other way.
Then disaster! My step-father's glance had strayed that way and he'd seen the lit match! I was the only one who slept on that side of the lounge room; my sisters were on the other side and fast asleep! One rather sore bum later I retired to bed.
Well, Galileo suffered rather more than I did...