I will work harder!

These are words from my little boy. He finished his 'workbook'(white folder) and asked for more, so I let him try some of Joy's worksheets (green folder). He loved doing sister's work and I am quite surprised that he could do the counting worksheets with no mistakes. Once, he did purposely circle the wrong number after counting aloud and laughed at my response. Cheeky boy. While he was doing Joy's workbook, he told me that sister is not good in these worksheets, and he will work harder (to do them).
I am not sure I am happy with his words. To me, it means that he is observant enough to sense my frustration when teaching Joy, and he is eager to please me by working hard at things that he knew that sister is not yet capable. But on another perspectives, my little boy is a fast learner and very observant. I guess these are good foundation for him for a bright future.