Harry usually tried to avoid lying directly, but he knew how to do it. The trick was to temporarily make yourself believe that the lie is the truth, and then to behave exactly as you would if you were being honest. So he quickly constructed the fiction in his mind, then met Snape's eyes, just as he would if he were telling the truth. "I have no need for any others, sir. I only carried that many because three of them were designed to be thrown; otherwise, two would have sufficed."