Marinades keep grilled foods juicy, tender, and flavorful, but in order to do their job, they must include a few key components. Acidic ingredients, such as vinegar or citrus juice, break down proteins, allowing marinades to penetrate meats. Enzymes found in certain fresh fruits, such as pineapple, also break down proteins, but they only act on the surface. Marinades made with such fruits help a tasty crust form when you're grilling thick steaks or other hefty cuts of meat. Since acids alone can toughen meat, marinades also need fat. Olive oil and other fats help seal in moisture and season foods. Once an acid or enzyme and a fat are in place, the other seasonings are up to you.