Carl calls in sick to work.
His boss says "I know you're not sick, now get your arse to work!" Carl replies "No, I'm very sick, I'm not coming in".
The boss is pissed off, so at lunch time he decides to go to Carl's house and bust him.
When he arrives, he peers through the window to see if Carl is in fact there.
To his surprise, he sees Carl on the bed hammering a girl doggy style. The boss runs to the front door and practically bangs the door down with anger knocking on it.
When Carl answers the door, his boss is livid "I knew you weren't sick, you're fired!"
Carl replies "I am sick. That's my sister!"