We call it 'Rhyme'

First case:

You want custard
I give you mustard
'Cos it's a standard
For a bastard...

(22 Aug 2009, 11:11 WIB)

Second case:

My sweet bjootie (=beauty)
When you shake your bootie
You look so cjootie (=cutie)

Is it maybe
My Tweety Bayby (=Baby)
'Cause you are a...laaydy? (=Lady)

(22 Aug 2009, 13:01 WIB)

Ultimate case:

The sky is blue, and I love you
The sky is purple, I'd love an Apple
The sky is red, just turn your head
The sky is brown, don't let me down
The sky is yellow, come on, fellow!

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I hope it is inspirational for you....:-D