Scientists now know why coffee rings have dark, well-defined edges, as seen in the image above. The research finding may have implications on the development of inks and paints. Marina Dominguez/NPR hide caption

itoggle caption Marina Dominguez/NPR

Linguist John McWhorter says sign languages function in ways that no other languages do. But, he says, like all spoken languages, they also morph over time. iStockphoto.com hide caption

itoggle caption iStockphoto.com