Chemistry
I always remember a substitute teacher we had initially for science lessons, who ended up doing everything. he was an ex-army medic who essentially had studied those "how to remember everything" books to the letter and so specialised in...well pretty much everything. i had him as a maths tutor and he really helped out, when he could stay on subject. He was a guy who could literally learn a conversational level language in a week. Scary stuff.
In one of our chemistry lessons he told us a rhyme that I'm sure many of you have heard, and those that havent and understand chemistry will appreciate the joke I'm sure:

Billy was a chemist,
But Billy is no more.
For what he thought was H20
Was H2S04
(Brighton RAWK at the tone, the time will be:, Fri 18 Mar 2011, 11:13, 5 replies)

And for ten points...
... decode:

A mosquito was heard to complain;
"A chemist has poisoned my brain.
The source of my sorrow
is para-dichloro-
diphenyl trichloroethane"
( Ugi , it's a game of give and cake, Fri 18 Mar 2011, 16:24, closed)

Is it RAID?
DDT innit.
And Sulphuric Acid?
( His Popiness the Hole Invaded Poland on, Sun 20 Mar 2011, 22:18, closed)

admiteddly i cannot comment on the mosquito's plight, however H2S04 is indeed Sulphuric Acid :)
(Brighton RAWK at the tone, the time will be:, Mon 21 Mar 2011, 9:46, closed)

DDT it is..
Dichloro-Diphenyl-Trichloroethane.

Not actually a completely correct or unambiguous chemical name but not incorrect and it's where the abbreviation came from, I believe.
( Ugi , it's a game of give and cake, Mon 21 Mar 2011, 10:03, closed)

I'm such a nerd.
My head is full of shite. However, if amongst that shite had been Harold MacMillan's real first name, I could have been a nerd with quite a lot of money.
( His Popiness the Hole Invaded Poland on, Mon 21 Mar 2011, 21:22, closed)