Sharks are literally wired for hunting. The finned predators of the high seas are equipped with a special sense called electroreception that allows them to home in on prey with deadly accuracy. Other members of the elasmobranch fish family -- rays and skates -- also share this trait, but sharks' electroreception abilities are the most finely tuned.
Electroreception simply means the ability to detect electrical currents. What does electricity have to do with sharks' underwater habitat? Any muscular movement or twitches in living animals and fish create small electrical currents. At hospitals, electrocardiogram machines track the electricity resulting from our heart beating.
Open air does not conduct this electricity away from our bodies, but thankfully for sharks, salt water does. Salt in salt water contains sodium and chlorine ions. Ions are particles that have an electrical charge because they have lost or gained an electron. In water, these sodium and chlorine ions in salt separate and move freely, transporting electricity.
You can compare this to how batteries work. It's set up like an electrochemical cell that separates the negatively and positively charged ions. When connected by a wire, those opposite charges attract, meaning the positive and negative particles flow toward each other to pick up or drop off electrons to become stable again.
A similar thing happens in the interaction of living cells and salt water. Because fish cells have a charge different from the saltwater solution in which they swim, the contact creates a weak voltage in the same way as a battery. Sharks can sense the tiniest changes in this electrical current, down to one-billionth of a volt [source: Fields]. If two AA batteries were connected 1,000 miles (1,600 kilometers) apart, a shark could detect if one ran out [source: Viegas].
How can sharks do that? Read about the part of sharks' bodies that regulate this unique internal homing device on the next page.