Or...
Just use a UV pen.
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Wed 16 Sep 2009, 11:43, 2 replies, latest was 15 years ago)

But...
...where's the fun in that?
(Che Grimsdale Best before:, Wed 16 Sep 2009, 12:01, Reply)

Che has a point
combining practical jokes with a wank is win-win.
( sandettie light vessel automatic New Twitter - @bollocksreally, Wed 16 Sep 2009, 12:52, Reply)

I went through a stage
where I had a UV pen, and I used it to draw a beard and glasses on the Queen and a speech bubble on every character on the back so they were saying expletives. On every single banknote I had until the pen ran out.

Seeing people react to the Queen is one thing, but when they saw Michael Faraday and Charles Dickens saying 'Pissflaps', their face was a picture.
( sandettie light vessel automatic New Twitter - @bollocksreally, Wed 16 Sep 2009, 13:01, Reply)

Should have guessed...
...you'd been there before me!
(Che Grimsdale Best before:, Wed 16 Sep 2009, 13:58, Reply)

ahh but
a UV pen is one thing, but using spunk is a whole different league. When you hand someone a banknote, you get the chance to have a titter and maybe tell them whatever it is that's fluorescing.
( sandettie light vessel automatic New Twitter - @bollocksreally, Wed 16 Sep 2009, 16:56, Reply)

I want a UV pen now.
NOW!
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Wed 16 Sep 2009, 14:32, Reply)

you have one
between your legs
( Fitznicely into a thimble, Sat 26 Sep 2009, 4:36, Reply)