What an overworked phrase! Catchy, but overworked. We're not talking about when to take a Viagra or Cialis. Professional photographers have known for decades the kinds of photos they capture when the serendipity springs up before their eyes. They capture the moment because they press the shutter at just the right mille-second. Sometimes such moments can be planned, and sometimes they cannot. Sometimes they are just there, and you have to snap it without thought for composition or lighting or any of the other elements we know are important for quality photography.