Echo Location/Ears to You

S1 E29: On a camping trip, the kids discover that Ari is really good at playing Marco Polo. Even without saying "Marco," a blindfolded Ari can easily figure out where Olive and Elinor are. How does he do it? Then, while playing at the lake, Elinor and her friends notice something very interesting... they can't see ears on some animals. That's sooo interesting! How do they hear?