Why wont OnCollisionEnter2D work?

Questions & Answers
1

Both objects have rigidbodies and colliders. They will in fact collide (my bullet pushes the other object away), but the debug won’t work.

void OnCollisionEnter2D (Collision2D col){
	Debug.Log ("hit");
}
2

Are you using RigidBody2D and Collider2D? The 2D and 3D physics systems are completely separate and cannot interact.