Why is Grocery Shopping harder for me too?

I was in Cub Foods Monday afternoon trying to complete what I had assumed would be an easy task....grocery shopping. My tween and I were getting along, we had the Oreos, the Tombstone pizzas and the Sunny-D in the cart and were on our way out without a hitch....until I tried to check out. Our cashier started out pleasant enough...a middle-aged woman with a warm smile....but she quickly got on my nerves. Our small talk started out with basic chit-chat about coupons and whether or not I had enough Mac and Cheese to get the 25 cent discount that Kraft was offering. But as any parent of a bi-racial child knows, there is a shift that often happens in the course of an otherwise innocent conversation that comes on so suddenly that you wish you had the powers of the animals to sense a tsunami. One minute all is calm and pleasant, and the next you find yourself broadsided with disaster. You even start to look around for allies, but realize they sensed the shift before you did and they ran for the hills -- or if they stuck it out it makes them too uncomfortable to even make eye contact. In hindsight of course, I can identify the shift. It was when the woman asked why my son wasn't in school. I should have realized then and there and by answering her it gave a naturally busy-body personality the go-ahead to ask me more inappropriate questions. When I told her "he just doesn't" she immediately turned to him and asked, "So what nationality are you anyway?" All these years later and for as many times as this has happened, I have to admit I still don't have a plan of attack for ignorance. I guess I just keep hoping that this will be the last time and I won't have to plan for another disaster like this one. I shot her a look of daggers-that very clearly-even if non-verbally-told her that this conversation was OVER! I put my arm around my son and literally turned both his and my back to her and struck up a new conversation with a stranger who was bagging her own groceries. This apparently, instead of discouraging the cashier as it was meant to, encouraged her to tap my son on the shoulder and ask "so do you speak Spanish then?" I stood tall, glared straight into her beady little eyes and asked, "What is it about him that makes you think he speaks Spanish?!?!" I dared her for at least 30 seconds to respond with something like "because he's brown" before I turned and tried to left the store. But she ran after me (does this lady quit?!?!?) to say, "you have a great kid there." And I'll be darned if I didn't read between the lines..."Where did you adopt him from?"