OnMouseEnter is called when the mouse entered the GUIElement or Collider.

	// Attach this script to a mesh to make
	// it red when the mouse is over the mesh
	function OnMouseEnter () {
		renderer.material.color = Color.red;
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnMouseEnter() {
        renderer.material.color = Color.red;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnMouseEnter() as void:
		renderer.material.color = Color.red

This function is not called on objects that belong to Ignore Raycast layer.

OnMouseEnter 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.