Ok.
The episode is all about deception.
The argument is:-
If you have ended up with the Q over your right eye so you can see it, you see the world from your perspective.
So therefore you are a pretty honest person.
The lefties think of the world as though somebody is looking at them, so they...
without thinking about it, trace a capital Q onto your forehead.
The key thing is over which eye have you put the tail of the Q?
The Radio 4 reasoning will be with you shortly.