How to "cut" a piece of glass in more parts??

You would need to have the glass in full and the glass in pieces. you can attach rigidbodys to the glass in pieces and put it into a Prefab.

Then when you want to break the glass, you would delete the in tact one and replace it with the broken glass prefab like so.

BrokenGlassPrefab : GameObject;

function OnCollisionEnter(other : collider){

if(other.gameObject.CompareTag("head"))
{
var BrokenClone : GameObject = Instantiate(BrokenGlassPrefab, transform.position, transform.rotation);
Destroy(gameObject);
}
}

put that on the in tact glass and place the prefab in the broken glass variable.

Crazy idea, I’ve not tried it, but what if your glass consisted of many triangles (like the standard Plane object), and when hit, you’d edit the mesh to split it into several smaller meshes. Like say you wanted 10 shards in the result, assign each triangle a random number from 0 to 9 and create 10 new meshes, copying the verts and faces to those meshes. Some of those might contain several triangles, which, when hit again, would do the same type of thing.