Your answer should have been "because it's called a SLACK tub, not a busy tub, and my tongs needed a break." Then just stare at him like that made perfect sense. If he pushes back, tell him it's for the same reason you don't put cold water on hot iron unless you WANT it to crack. Half the stuff we do in the shop is just habit passed down from some guy who learned it from another guy. Next time just tell him the tongs get thirsty.