On his European television series, Royle presented this as a scientific experiment where drops of beer placed on the top of bottle caps would bubble up if there was a coin hidden underneath them.
On stage he has used it as a demonstration of "muscle reading" without the need for any kind of skill in muscle reading.
It can also be used as a lie detector test or proof of your mastery of body language and eye assessing cues.
In short five bottle caps which can be borrowed from behind the bar are examined and confirmed to be 100% ungimmicked.
A volunteer places a small coin under one of the bottle caps while your back is turned or even while you are out of the room.
Yet instantly you know exactly which bottle cap the coin is under and it can be repeated time and time again without anyone ever discovering the cheeky, simple, but yet highly effective and baffling secret.