Two Objects - Contraction?

I have two objects. One sphere is moving left and one sphere is moving right. Now, when they’re too close (when colliders touching), they should contracted and destroyed.

How can I solve this?

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There are many ways to do this. I believe you mean “collision” as opposed to “contraction”.

Simplest way would be something like:

void OnTriggerEnter (Collider col)
{
    // Do some check here to make sure it's the object we want to destroy on
    if (someCheck)
    {
        Destroy(gameObject);
    }
}

We’d need more information about the exact behaviour you want.

Really you should just look up collision detection. There are lots of official resources on the Unity website. Start here: http://unity3d.com/learn/tutorials/modules/beginner/physics/colliders-as-triggers

You may find it helpful to do some more intro tutorials before asking more questions on forums like this.