The Rigidbody we hit (Read Only). This is null if the object we hit is a collider with no rigidbody attached.

	// Make all rigidbodies we touch fly upwards
	function OnCollisionStay(collision : Collision) {
		// Check if the collider we hit has a rigidbody
		// Then apply the force
		if (collision.rigidbody) {
			collision.rigidbody.AddForce (Vector3.up * 15);
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnCollisionStay(Collision collision) {
        if (collision.rigidbody)
            collision.rigidbody.AddForce(Vector3.up * 15);
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnCollisionStay(collision as Collision) as void:
		if collision.rigidbody:
			collision.rigidbody.AddForce((Vector3.up * 15))