Blu-Tac
Have a work colleague who is always fucking about with a blob of Blu-Tac? Maybe they use it as a stress relief ball thing that used to be popular 10 years ago, maybe they use copious amounts of the stuff when they don't really need to.

When they're not about, preferably at lunchtime when they're gone for a while, collect up their Blue-Tac, spear it onto a paperclip and set fire to it.

It will burn steadily much like a candle wick and whilst doing so turns from blue to white. Eventually, whatever it is that is flammable will run out and the flame will go out.

As the 'white tac' as it has become cools down, it turns back blue again. It's like magic.

When it's cool enough to touch, ever so gently take it off the paperclip and place it back on your colleague's desk. It still looks exactly like the original piece of Blu-Tac except now it has different properties.

It has become a chalky substance which is more crumbly than a stock cube and consequently when the colleague picks it up, it will fall apart between their fingers.
( sandettie light vessel automatic New Twitter - @bollocksreally, Wed 27 May 2009, 11:53, 5 replies, latest was 15 years ago)

You seem to have far too much spare time at work.

( Bob Todd whee, Sat 30 May 2009, 0:41, Reply)

That's amazingly pointless
and yet quite, quite funny...
(mr-lizard Hell's teeth man!, Sat 30 May 2009, 2:59, Reply)

that has both
wooo and YAY!
(knackerz is skinning up, Sat 30 May 2009, 5:08, Reply)

I really want to try this....
pity the fire alarms in my halls are more excitable than a spaniel on speed
( Tedium Alduce me to interlow myself., Sun 7 Jun 2009, 14:39, Reply)

I'm seriously starting to worry about you
But in a nice way.
(Che Grimsdale Best before:, Tue 9 Jun 2009, 16:54, Reply)