Rock smashes scissors. Scissors cut paper. Paper covers rock. The rules behind the favorite game of schoolyard kids and adults deciding who takes out the trash are pretty simple. But they also represent a kind of logic problem. Four-year-olds can learn the rules, and so can chimpanzees—but the differences in how kids and apes become proficient reveal a little about how their minds work.
The relationship between the three items in rock-paper-scissors is circular. There isn’t a straightforwar