When I was young I was taught the art of spanner wielding by a kindly neighbour who had picked up his mechanic skills working on aircraft for the RAF during his National Service. Two of them were assigned the task of performing routine maintenance on a huge pile of transfer boxes (for what I do not know). This involved replacing worn washers, bearings and so forth, checking tolerances, lubricating and moving to the Done pile.
At the end of the day, my man cleaned his work station (he was meticulous in these matters) and looked over to his workmate. He had a pile of washers, springs and other bits and bobs, both new and used on his bench. He cried, "Spares!', then picked them up and threw into the bins of new parts.
My neighbour learned a lot about how far to trust the workmanship of others from that and was keen that I learned from his experience.