My grandson (then 8) once (only once!) told me he'd show me how smart he was.
I said, "Ok, show me."
He then asked me to give him the calculator so he could show me how smart he was.
After suppressing a big, hearty laugh, I responded by saying, "That won't show me how smart YOU are. It'll show me how smart the calculator is."
He left the room without another word.
The rest of the story: He has Aspergers Syndrome and truly is very smart. Mom and Dad thought he 'learned things differently', that he didn't need to do all the work other kids needed to do to learn. Since I was not a 'true believer' in undemonstrated abilities, I wrote down some simple math and addition problems for him to add and subtract.
Again, he asked for the calculator. Again, he was told no. He confidently began adding the two numbers, which were hundreds, from left to the right, and insisted that his answer was right. After wrangling over whether his answer was right or wrong, I handed him the calculator and he 'figured' out that he was wrong.
The same thing happened with subtraction.
It took me 1.5 days to teach him how to do addition and math, and, like with most kids, it required repetition, repetition, repetition.