Counting Toes

Aileen and Joann were walking home one evening. Aileen was wearing her sandals while Jo was wearing her school shoes... 

Aileen: Jo, what's 6 + 7? 

Jo ( paused for a while) : Easy. It's 13.. 

Aileen: Well done! How did you do that? 

Jo: I've got 10 fingers. Then, I used toes..

Aileen (eeks): but you are wearing your shoes so how did you use your toes? 

Jo (wide-eyed, innocently) : I used YOUR toes...