First, lets look at the difference between theory and scientific theory. Definition of Theory An unproven conjecture; an expectation of what should happen, barring unforeseen circumstances In other words, you imagine something will happen and it may or may not, depending on circumstances you werent expecting. Definition of Scientific Theory: A well-tested concept that explains a wide range of observations. In other words, a well tested concept based on actual repeatable tests and observations. And how do we get to the scientific theory? We use a method known as, The Scientific Method. The following is a simplified description: Definition: The scientific method is a series of steps scientists take to acquire, test, and describe the natural world. Step 1: Ask questions in the form of a hypothesis Step 2: Look for patterns in observations Step 3: Formulate a theory Step 4: Design experiments to test theory So the question becomes, If you can observe it, then why is it only a theory? Appearances can be deceiving. Sometimes, there is no direct observation. Until we are able to create video such as this, we thought sea urchins and starfish were very slow moving and sedentary creatures. After speeding up the video, we now know they are extreme hunters with voracious appetites. Previously, this was only a "theory". [ame]http://www.youtube.com/watch?v=D3W4OCnHyCs[/ame] Some events are so slow moving, they cant be taped but only studied by looking and understanding the available evidence. Two examples are evolution and plate tectonics. Evidence proving plate tectonics are mountains and faults, ocean floors raised thousands of feet above sea level and the tiny observations from satellites, geology and many other bits of example. Evidence proving evolution are fossils, where the fossils (geological location) are found, genetics, Disease, birth defects and fetal development among others. Any alternate theories must be scrutinized by the methods listed above.