A call to OnMouseEnter occurs on the first frame the mouse is over the object.
OnMouseOver is then called each frame until the mouse moves away, at which point OnMouseExit
This function is not called on objects that belong to Ignore Raycast layer.
This function is called on Colliders marked as Trigger if and only if Physics.queriesHitTriggers is true.
OnMouseOver can be a co-routine, simply use the yield statement in the function. This event is sent to all scripts attached to the Collider or GUIElement.