There is nothing wrong with crying, but that doesn't mean you want to do it in every situation. If you need to compose yourself quickly, a quick pinch in a couple of places on your body might help you refocus and stop the waterworks.
Photo by Anders Ljungberg
Everyone has those moments where they can feel the tears coming in, but don't want to draw attention to themselves. You might be at work, in class, or in a store. Wherever you are, if you start to get teary-eyed, Melissa Dahl at NYMag's Science of Us blog suggests a hard pinch in the webbed portion of skin between your thumb and pointer finger, or at the bridge of your nose (which can help wake you up too).
The pain offers your brain just enough distraction to get your mind focused on the immediate moment, not whatever sad thing you may have been dwelling on. You don't want to pinch too hard — and it may not work for highly emotional states — but it might be just enough to buy you some time until you can be in a more comfortable setting.