Draw with me~

Look closely. She cut off her non-drawing arm.

Not only that, but if you pay close attention, the boy is left-handed and the girl is right-handed. Therefore, her cutting off her left arm wouldn't be a problem since she can still draw with her right.

(Also, at 1:18, there is an inconsistency; at first the boy puts his right hand up against the glass, but when it cuts to the next shot, it's his left hand against the glass.)

The more I watch this piece, the more I love it.