creases helping the hammerhead to feed. It is also believed that the hammerhead may be able to use the Earth's magnetic field as a source for navigation. Some hammerheads migrate a lot and may rely on this built in compass sense to guide them in the open ocean. Another use for the hammer is to enhance maneuverability. The hammer's similarity to a hydrofoil seems to explain its usefulness for maneuverability and improved lift. However, this theory has not been tested. Sharks generally have a small brain in comparison to their body weight. Among sharks hammerheads have a relatively large brain-body weight ratio. Sharks differ form most other fish in several ways. Sharks have a boneless skeleton made of cartilage that is a tough elastic substance. Most sharks have a rounded body shaped like a torpedo. This shape helps them swim efficiently. Hammerheads are especially good swimmers because of the hydrodynamic function of their head. All sharks are carnivorous. Most eat live fish, including other sharks. Most sharks eat their prey whole, or tear off large chunks of flesh at a time. They also eat dying animals. Hammerheads have definite food preferences. Their elongated head may help them locate the prey they prefer. The Great Hammerhead likes to eat stingrays. This was observed when the stomach contents of a hammerhead were examined and stingray spines were found. Stingrays are usually difficult to detect because they are partially buried in the sediment. Yet, the hammerhead is capable of finding them because they can swim close to the bottom swinging their heads in a wide arc like a metal detector. Sharks reproduce internally. Unlike most fish sharks eggs are fertilized internally. The male shark has two organs called claspers which release sperm into the female where it fertilizes the egg. In many sharks the eggs hatch inside the female, and the pups are born alive. Other species of sharks lay their eggs outside.