"I've gathered a lot of facts about Michelangelo and Angel. And I've let them grow inside me for a long time. Now I feel that I know." (9.252)
And that's the beauty of art: it sticks with you and grows inside of you. It's not just common knowledge, like math or botany or science. It's something that's beautiful and unexplainable, but it's obviously done a lot for Mrs. Frankweiler in her long life.