Cook

Attach particle to another GameObject's action (e.g. Projectile GameObject)

1. set the particle GameObject to be a child GameObject of the Projectile
2. get the particle from within the Projectile's script:
   • public ParticleSystem explosionParticle;
3. call Play() on the object: explosionParticle.Play(); from within the condition that would trigger it. (e.g. on collision of projectile)
4. attach the particle GameObject to the Projectile for the Explosion Particle param.

// ...
public ParticleSystem explosionParticle;
// ...
private void OnCollisionEnter(Collision collision) {
    if (collision.gameObject.CompareTag("Ground")) {
        explosionParticle.Play();
    }
}