Oh man, just remembered another good one.
I was a big fan of Beavis and Butthead as a child, even though I realize now that I didn't truly "get" some of the jokes. For instance, when they talked of "morning wood," I thought they meant your whole body feeling stiff when you wake up. Anyhoo, I was at my grandparent's house, and I come downstairs for breakfast. Everyone asks how I slept, and I say, "Really good, I have morning wood though..." as I start stretching my arms. I was 6 I believe. I still get hell for that from my grandfather.