EFFECT: C0D3X
You ask someone to remove an English £20 note from their wallet/pocket/purse, as you do the same. You show the front of your note and explain that both notes are exactly the same on the front (colour, text, value, material and shape). However, each note has its own individual serial number, which means that they are actually all unique. You briefly show the serial number on the back of your note, before putting it back in your pocket. You tell your spectator not to show you the back of their note, however, when you look away, they are to memorise the first four digits of their serial number. You explain that it would be too hard and unfair for them to memorise the whole serial number, which consists of around ten digits. The spectator returns the note back into their pocket and you turn around. You then proceed to read the spectator’s mind, and without any prompting, identify all of digits that the spectator is merely thinking of!