If there's one thing school children should be congratulated for, it's coming up with blue lyrics for the songs in the hit parade, often mere seconds after the release of the tune.

Back in the heady days of 1993 Ace of Base released "The Sign" which was transmogrified into "Your Mom"; the words to which are:
I saw Your Mom,
She opened up her legs and said 'come on'.
Life is demanding,
When you're doing it on the landing.

Brilliant, I'm sure you'd agree. Obviously you couldn't let someone sing this to you unpunished which led to someone coming up with the blocker song, "Your Dad", which replaced the fist two lines with:
I saw Your Dad,
He opened up his legs and said 'not bad'.

This, in turn, was blocked by the fact being pointed out that having your opponent's dad opening his legs on the landing and appraising you for sexy purposes was pretty fucking gay anyway.

I saw your mum
She opened up her legs, I gave her one.
It wasn't funny
But she needed the money.

If shagging your friend's mum was funny, or if she was independently wealthy and just whoring herself for kicks, you could substitute the last two lines with

It was fantastic
Totally elastic!

The lack of an alternative for well-to-do mums with bucket fannies is presumably down to their penchant for consulting with plastic surgeons and getting an 'elastic' nip n tuck to the old bacon sandwich.

I think you mean;
I fucked your Mum
I opened up her legs and made her come
She was outstanding
Especially on the landing.
Then move onto the father, remembering that it's not gay to fuck another boy's father;
I fucked your Dad
I fucked him, sucked him, played with his gonads
I felt his power
When we were in the shower.
(Let me try! Cough - here we go...
I snogged your gran,
I mopped up her womb juices with a naan.
I fisted said womb
In her filthy bedroom - Log)

David Chiswell's mum was a bit of a goer and she done it how she liked it. Singing this steamy tribute to her subtle charms in his face was the least we could do.
I fucked your mum
I opened up her bum, I fucked your mum
She was demanding
So I fucked her standing

After school we would often drink a potent concoction known as "bine" which was half beer, half red wine. After drinking a large amount of bine my buddy spent the evening singing:

I drank the bine,
I opened up my mouth,
I saw the bine.

When he says "saw the bine" I think he actually meant "regurgitated the bine", but that didn't scan so well; and the song never evolved beyond a repetition of this perfect quasi-haiku.

You should have seen the state of us after a heavy night on the Pimmto.

To the tune of 'Wannabe' by the Spice Girls:

If you wanna be my lover,
Sex is 50p.
Condoms are one-fifty,
Buy one get one freeee!

Every time I see a 'Buy One Get One Free' offer I get the urge to sing that song.

You should go right ahead. It'd probably brighten up some old sod's Tesco shopping trip anyway. - Matt