Harry Potter was a small child, short and malnourished. No, this wasn't his fault, but the fault of his aunt and uncle. He had been left on their doorstep as a baby, and been treated badly ever since. He did their dishes, clothes and and any chore they wished, he was their Slave. They beat, whipped and tortured him regularly, when anything wasn't done to their standards, or really whenever they wanted to.
Harry had learned to show no emotion around them, to give nothing away.
Harry was rarely allowed to leave the house, and on the rare occasion that he was, he was sent out to do the shopping.
Today was one of those days, he was free.