My son was a thumb sucker. One day when he was 2 he wanted to sit on his grandpa's lap. His grandpa told him he could if he wouldn't suck his thumb. He said, "Ok grandpa."
He hadn't been on his grandpa's lap very long before his thumb went in to his mouth. His grandpa said, "I thought you said you wouldn't suck your thumb." To which my son replied, "I wasn't sucking my thumb grandpa, I was just chewing my fingernail."