The operation of the notional machine presents a hidden but crucial process in students' understanding of introductory programming. When students trace though code, simulating the operation of the notional machine, this hidden operation becomes evident. When students sketch this trace by physically drawing it, the operation is also visible to peers, tutors, and teachers as well as to the students themselves. Increased accuracy on code reading problems has already been found for students who sketch and trace. I want to explore whether sketching and tracing helps others better understand what a student knows about the notional machine.