A childhood belief about speaking

When I was in Kindergarten, I had the thought implanted in my mind "Say no to things you don't want, and say yes to things you do want". Well, I hadn't really learned the concept of "Would you mind?". One day my friend came up to me during class, and asked "Would you mind if I sat with you?" I responded "Yeah." because I thought saying 'Yes' meant 'Yes, she can sit here'...I was suprised when she suddenly got up & walked away o_o; One day during lunch, my teacher walked over to me, smiled, & said cheerily "You seem to be enjoying lunch, Melissa! Would you mind if I sat with you?" and as usual, I said "Yeah, sure!" cluelessly. Her face changed to one of a shocked & confused look, and she asked, "You don't want me to sit with you?" I looked at her as if she were crazy, and replied, "No! I just said yes!" And then she smiled, and corrected me.