At the onset, I presumed that sharks are mammals, simply because they give birth to young ones (known as pups) and do not lay eggs. But then, not all sharks give birth to pups, some do lay eggs. Of all the species, 70% of the sharks bear live ones and the remaining 30% lay eggs. While that may make many people assume that sharks are mammals, the fact is that they are fish.

• Mammals: Mammals are warm-blooded living organisms belonging to class of higher vertebrates whose bodies are covered with hair, who give birth to live young ones, and feed them with milk from mammary glands.
• Fish: Fish are mostly cold-blooded aquatic vertebrates that may or may not have scales, breathe through their gills, and are equipped with two sets of paired fins and several unpaired ones.

The breathing mechanism of mammals and fish is different. Marine mammals or sea mammals, like whales and dolphins, draw air into the lungs by breathing through the 'blowhole' (similar to nostrils in terrestrial mammals) on the top of their head. For this, they have to come to the surface after regular intervals. These mammals hold their breath to keep the blowhole closed to stop water from getting into the lungs. Fish, on the other hand, have gills which help them breathe oxygen directly from water without any need to surface. The fish takes water through its mouth, pushing it over the gills and back out through the gill slits. The oxygen enters the blood vessels after being filtered by the gills. Sharks do not have lungs, but have between five and seven gill openings. The skin is another differentiating point among warm-blooded mammals and cold-blooded fish. Marine mammals are warm-blooded which means that they can keep their blood warm and body temperature steady and warm (enough) despite the water they swim in, hence have smoother skins. Whereas, fish have skin covered in (smooth or rough) scales. Fish are cold blooded, which means they cannot keep their body temperature steady, hence can change as per the climatic conditions of the water they are swimming in. Sharks have rough tooth-like scales covered by thousands of tiny teeth called 'dermal denticles' which not only protect their skin from parasitic damage, but also helps them move really fast in water. Marine mammals either have one dorsal fin or none at all, whereas fish have more than one dorsal fin, and pelvic and/or anal fins too. Sharks, depending upon their species, have first, second, pectoral, pelvic, anal, and/or caudal fin which they use for stabilizing, steering, lifting, and for thrusting ahead. Shark fins do not allow them to swim backwards, hence, they can only drift away from objects directly in front of them, instead of moving sideways. Once you are well-versed with the basic difference between mammals and fish, it becomes more than obvious that shark is a species of fish - not a mammal as many people believe it to be.