My third hand

At dinner time we often do fun stuff like having the kid do some math or stuff like that. Mathieu who has just began kindergarten was tasked to add numbers. Now he's really good at adding numbers that sum to 10 or less but he sometimes still needs to count on his fingers. But this time he numbers were I believe 9 + 3. He put his two hands in the air, looked for a few seconds to the right of his right hand, then at his right hand, then at his left and went 12.

He then proceeded to tell us he used his invisible third hand to get to 12.