How does Mrs. Loftus figure out that Huck is not a girl?

She asks Huck for his name again. Huck replies, "M -- Mary Williams." Realizing he has made a mistake, he says that his full name is Sarah Mary Williams. The woman tosses Huck a hank of yarn and Huck closes his legs together and catches it. The woman immediately knows that he is a boy and not a girl because girls will catch things in their skirts by spreading their legs apart, whereas boys put them together to grasp an item.