We got a great wee chat from our biology teacher once about how important it was to come and talk to him after class if there was anything we didn't quite grasp in the lesson - his case in point...
Having covered the thorny issue of 'reproduction' he asked a class if they had any questions - boy puts hand up
"When people have sex how do they know?"
So the teacher starts to recap on conception and the first signs of pregnancy and....
"No - I mean how do they know they've had sex? If they're asleep?"
Kid had taken the whole "Going to bed with.." and "Sleeping with..." to mean both participants were fast asleep during the whole act.
Silly boy. He would of course later learn that only one of the participants is likely to be unconscious.