Talking in letters

Now that Sam is learning how to read and write he thinks its hilarious to talk in letters instead of words.  Here are some examples:

Me: Do you want milk or water?

Sam: W.

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Me:  Sam do you want a waffle or cereal.

Sam: C.

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Sam: Mommy I forgot U.

Me: U?

Sam: Yep. U.

Me: Underwear????

Sam:  yep.

*this was on the way to school!

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Me:  Sam do you want a snack?

Sam: Y

Me: What do you want?

Sam: A

Me: Apple?

Sam: Y

I have gotten good at deciphering what he means - Mark however is totally confused!!