When we write a standard C# program, we would use the Main() function as our starting point. In Unity, we use the Start() and Update() functions as our main sources of flow control; these functions are part of the MonoBehaviour class.

MonoBehaviour is the base class of all Unity scripting. All classes derived from MonoBehaviour are required to be attached to a GameObject to run. MonoBehaviour also has a set of events or functions each script responds to.

We have touched on some of these events in previous chapters:

• Start: This is called on the frame when a script is enabled, often used for setup.
• Update: This is a method that is called every frame.
• OnMouseDown: This is a method that is called when the user clicks the mouse button. This method is tied directly to the MonoBehaviour base class.