How to make kids quiet

You know how teachers raise their hand straight up when they want kids to be silent and essentially shut up. Except it half works because kids just raise their hands and talk simultaneously. And theres the clap the rhythm method which is more effective than the raising your hand method but yet again kids just start talking after clapping the rhythm.

I don't know if this method would work but why don't they get kids to put both hands over their mouths instead? That way you'll get their attention and anything they say will be a muffled sound. Except kids find that kind of thing funny and you'll get a massive burst of laughter instead, depending on the age group of the kids.

So what is the best method to make a large group of kids shut up and stay that way?