Harrison investigates the stuff that holds us in: skin. With help from a special-effects make-up artist, he finds out the science behind scabs and bruises, and even learns how to make cool fish-monster scales. In the Field: Harrison goes to the beach with a skin scientist to test how sunscreen can protect UV-sensitive Frisbees from turning purple.

The Big Question: "Why don't humans have scales?" 1) MY GREAT CHALLENGE: Are parts of our skin more sensitive than others? Blindfolded challengers have to identify mystery objects rubbed against their foot, leg, and hands. 2) IN THE FIELD: Harrison goes to the beach with a skin scientist to test how sunscreen can protect UV-sensitive Frisbees from turning purple.