On the day of the exam, the professor has a table with five paper bags on it. Under each bag is a model of a bird. The professor lifts the first bag to reveal only the legs, and tells the class to ...

Back to feet: Smaller birds with shorter legs, particularly species favoring trees, hop on land. Birds with longer legs are more likely to walk. Some, like American robins, do both, depending on need.

A C.T. scan of the foot of a 99 million-year-old bird whose feathers, not visible on the scan, were preserved in amber. Credit... Lida Xing et al.