Lukas just did his first double-digit subtraction!

Lukas (5 yrs 5 months), who is currently in Kindergarten, just did his first double digit subtraction. Using fingers only. I asked him about his book reading assignment- if he needed to read 18 books, and only read 9 so far, how many more books did he need to read? The incentive was that if he did it correctly, he could watch some toy review movies about Treasure X. He did it right, on his first try!!!