Answer key (IMAGE) University of California - San Diego Caption Answer key to the test, which was too difficult for the children to complete without cheating. Credit Courtesy Li Zhao, Hangzhou Normal University. Usage Restrictions Only with coverage of this PNAS paper. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.