The jack!

A school teacher notices a student is getting much better with numbers than he was just a few weeks earlier. “Your counting has improved,” the teacher tells him after class.
“Thanks,” the boy says. “My dad will be glad to hear that. He’s been working on them with me every night and weekend.” “Fantastic,” the teacher replies. “So here’s a quick quiz—what comes after nine?”
“Ten,” the boy replies enthusiastically.
“Right, and what comes after ten?” the teacher quizzes.
“The jack!” the boy answers.