Sam was still grinning, but there was a curve of lip which was far from any real humor, even that provoked by the practical jokes of a barracks bully.
Sam had been grinning for about 10 minutes.
mnli hypothesis: Sam had been grinning for about 10 minutes. premise: Sam was still grinning, but there was a curve of lip which was far from any real humor, even that provoked by the practical jokes of a barracks bully.