Sine is opposite over hypotenuse by definition. The length of the opposite side gets shorter and shorter as the angle gets smaller. When the angle approaches 90 degrees, the opposite side becomes almost as long as the hypotenuse and at 90 degrees it is the same length making the sine value 1. As the angle goes past 90 degrees, the opposite gets shorter again until at 180 it has zero length so sine is zero again. The opposite drops below the base of the triangle so sine is negative and eventually passes through 270 degrees (sine=-1). Between 270 and 360 the opposite side gets shorter and shorter so that at 360 sine=0. Then the process starts all over again. So every 180 degrees (π radians) the sine=0.