These unique, lumpy-looking clouds filled the sky over Stillwater, Oklahoma, following a thunderstorm in the area on Monday. The clouds' signature rounded shape has earned them the name 'mammatus clouds,’ after the Latin word for 'breast.'br » Sign up for our newsletter KnowThis to get the biggest stories of the day delivered straight to your inbox: » Subscribe to NowThis: br For more U.S. news, subscribe to NowThis News.