You need to know what triggers the event. For some events, the answer is obvious. For example, a mouse-down event occurs when the user presses the left button on the mouse and Windows sends a WM_LBUTTONDOWN message to the application. Upon receiving that message, a component calls its MouseDown method, which in turn calls any code the user has attached to the OnMouseDown event. 

However, some events are less clearly tied to specific external occurrences. For example, a scroll bar has an OnChange event, which is triggered by several kinds of occurrence, including keystrokes, mouse clicks, and changes in other controls. When defining your events, you must ensure that all the appropriate occurrences call the proper events.