Big A, Little A . . .

Could somebody please tell me why the creaters of the English language chose to make some upper-case and lower-case letters different in appearence. Cayla doesn't believe me that an "a" is an A, or a "b" is a B. In fact we have had many debates on this issue in the past few weeks. And tonight was no different. We were reading the twins' new favorite book at bedtime, Matthew A.B.C. by Peter Catalanotto. Right at the top of the third page ("Matthew A. is extremely affectionate."), Cayla spotted the "Aa" in the corner.

She pointed to the "a" and said, "That's not an "a"."
"Sure it is", I said, "it's a little "a"".

Cayla: It's not an "a"!
Me: It is an "a". It's a lower-case "a".
Cayla: It's not an "a"!
Me: It is an "a". It's a lower-case "a".
Cayla: It's not an "a"!
Me: It is an "a". It's a lower-case "a". (We've had this debate at least a few times a day for the past few weeks and I should get more creative in my response, but haven't come up with anything.)
Cayla: It's not an "a". It's an "o". (AAAHHHHHH - now we are getting somewhere!)
I gleefully turn to the "O" page (Mathhew O. stays outside.) and show her the "o". She proclaims it to be an "o". I turn back to the "a" page and say, "Look, this "a" is not an "o". It's an "o" with a line on the side." Cayla happily agrees, "It's an "o" with a line on the side!" "It's an "o" with a tail!" shouts Collin.
Me: Yes! An "o" with a tail! And an "o" with a tail is a little "a"!
Cayla: It's not an "a"!!!!!!!

I quickly turn the page (Matthew B. loves Band-Aids.) and prepare myself for the Bb debate. Luckily, her eyes immediately zip over to the next page (Matthew C. has friendly cowlicks) and the lovely Cc - finally a letter that actually has a big and little form that make sense.

(For the record, my favorite is "Matthew F. has a cat on his face." Collin really likes "Matthew L. leaks." (it's his nose and he is holding a tray of cupcakes - eeeeeewwwwww!).