Who said "I used to think I was smart."?

This quote is attributed to Charlie Brown.

I used to think I was smart. - Charlie Brown

"I used to think I was smart."

Charlie Brown Humor

"Good grief!"

"I got a rock."

"Rats! I did it again."

"Why is it that everything I do turns into a disaster?"

"Never underestimate the power of a good laugh."

"Laughter is the best medicine."

"A good laugh is the best medicine."

"Laughter is contagious and wonderful."