An analysis of mortar from the 600-year-old city wall in Nanjing (map) confirmed its mortar is a mix of powdered limestone and sticky-rice soup. The tip-off was the presence of amylopectin, a carbohydrate found in the rice, according to the study, led by Bingjian Zhang of Zhejiang University in China.

Historical-mortar expert Sedat Akkurt said via email, "Over time people have added many different materials to mortars (such as urine, blood, and eggs) so the addition of rice in China, where rice is such a big thing, does not surprise me."

Used in both sweet and savory Chinese recipes, sticky rice, or glutinous rice, is a short-grained Asian rice that becomes gummy when cooked.

(Related: "Stone Age Rice Fields