Unless you see the staff at the buffet replenishing a tray that's low on food, chances are the ones that are full-to-the-brim haven't been touched in a while -- which can be a bad thing.

These trays, dated between 6400 and 5900 BC, feature coarse grooves that facilitated the baking and removal of large loaves of bread, some weighing as much as 3 kilograms. The bread, baked in ...