I found this joke from http://www.jokes2go.com/.

No $

Dear Dad,

$chool i$ really great. I am making lot$ of friend$ and $tudying very hard.
With all my $tuff, I $imply can't think of anything I need, $o if you would
like, you can ju$t $end me a card, a$ I would love to hear from you.

Love,
Your $on.
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

Dear Son,

I kNOw that astroNOmy, ecoNOmics, and oceaNOgraphy are eNOugh to keep even an
hoNOr student busy. Do NOt forget that the pursuit of kNOwledge is a NOble
task, and you can never study eNOugh.

Love,
Dad