Devil Fruits are a big topic in One Piece. Whoever eats them, even a small bite, gains superhuman abilities. The first to be introduced is the Gum-Gum-Fruit. It is a Paramecia-type Devil Fruit which turns the one who eats it into a rubber-man. This has several advantages. The person is immune to cannonballs and gunshots, falling from high above is no problem and of course the body can be stretched in lots of cool ways. Who cares about being unable to swim?
Well, Luffy, who has eaten this fruit by accident, is not so happy with it at the beginning, because he wants to be a pirate. But he quickly gets used to his new abilities and of course he uses them to his advantage.