Untitled

using UnityEngine;

public class Grenade : MonoBehaviour {
    public float lifeTime = 3f; // How long the nade should last, make sure it's less than the particle life-time of the grenade emitter.
    public bool detachFX = true;
    public Transform scorchFX;
    float birthTime = 0f;
    LayerMask ignoreMask;

    void Start() {
        birthTime = Time.time; // Set time this grenade came into existance.
        ignoreMask = 1 << 8;
        ignoreMask = ~ignoreMask;
    }

    void Update() {
        if( Time.time - birthTime > lifeTime ) {
            Destroy( gameObject ); // Kill this object if it's life time has expired.
        }
    }

    // Handle grenade collision.
    void OnCollisionEnter( Collision collisionInfo ) {
        if( detachFX ) {
            foreach( Transform t in transform ) {
                if( t.name == "FX" )
                    t.parent = null;
            }
        }

        SpawnScorchMark();
        Destroy( gameObject );
    }

    void SpawnScorchMark() {
        Ray floorCheck = new Ray( transform.position, -Vector3.up );
        RaycastHit hitInfo;
        Physics.Raycast( floorCheck, out hitInfo, 10f, ignoreMask );

        if( hitInfo.collider ) {
            Quaternion randomRotation = Quaternion.Euler( 0f, Random.Range( 0f, 360f ), 0f );
            Instantiate( scorchFX, hitInfo.point, randomRotation );
        }
    }
}