A trainee joiner cuts the same joint forty times before anything gets built with it. Forty attempts into the offcut bin, in a workshop where the actual furniture is being made by other people at the other end of the room.
Nobody standing at that bench feels they are where they need to be.
Ten years on, the forty turn out to have been the whole thing — not the joint, but what the hands learned about the wood while making it.
This cannot be checked from inside. You can only notice that the stretches which felt like nothing have a way of turning out, later, to have been the part that took.