Taught her little brother subtraction, and now he takes her things and calls it maths

She spent part of the weekend teaching her 4 year old brother 'take away', with buttons, very patiently. He understood it perfectly, in his own way. This morning he removed two of her hair clips from the table and announced, 'Five take away two is three. Now you have three.' She is furious, and also, I suspect, a little proud.