aah okee, that explains it.
I was already wondering why you would jerk off at your friends place :P
And on vacation my parents went to do some shopping while my girlfriend and I stayed at the vocational home (don't know how you should call it exactly).
We got busy, first in the living room, then she said it might be a good idea to go to the bedroom in case they come home.
So we got to the bedroom and right before the climax of it all my little brother opened the bedroom door without knocking or whatever and said (before he looked inside the room) that they were home.
He has knocked on the door before entering ever since! =D