If someone stole a hide and trimmed it for use, he acquired it. Because it is now a usable utensil, it is susceptible to ritual impurity. If he only designated it in his mind as a tray, it also became a utensil susceptible to ritual impurity. But why should it be so, it hasn't changed?
Rav Yosef answered that it is a changed object because of a name change. It was a hide and is now a tray. This question remained unresolved for 22 years until Rav Yosef was promoted to Head of the Academy and received Divine assistance in resolving questions to enhance his reputation.
Art: Still Life - Two Glass of Red Wine, a Bottle of Wine; a Corkscrew and a Plate of Biscuits on a Tray by Albert Anker