When you aren't calm, you tend to hold your breath. Or sometimes you might be breathing shallowly and quickly. The fastest way to calm yourself is to focus on your breath and breathe deeply and deliberately, slowly if possible. Count your breaths. Notice how breathing feels in the body. In the stomach, the chest, the throat, the neck, the nostrils. Even with children, you can teach them to calm themselves when they're upset by just stopping whatever they are doing, breathing slowly, and counting ten breaths. So simple, yet so effective!