Structures are things that we build, that make life easier, like a house or a bridge. Harrison finds out how animals have been building structures for millions of years, as well as the best way to build structures, to keep them strong and sturdy, and realizes that humans could learn a lot from beavers.

The Big Question: "Why when I build my castle tall, it always falls down?" 1) MY GREAT CHALLENGE: Two teams of challengers build a bridge over dino-valley with marshmallows and pasta. Who will cross first, and who will be dino-dinner? 2) IN THE FIELD: Harrison visits some really cool tree houses, and discovers that tension can be very relaxing.