Sharks have no bones, their skeleton is made of cartilage.
Why it is true
Unlike most fish, sharks and rays have a skeleton entirely made of cartilage. Cartilage is lighter and more flexible than bone, which allows them to be more agile and use less energy. That is why complete fossils of sharks are rarely found, as cartilage decomposes more easily.
Source: Smithsonian
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.
Related facts
Sharks can detect a single drop of blood in an Olympic-sized pool.
Sharks have existed for about 450 million years, longer than trees.
Sharks have existed on Earth for about four hundred million years, making them older than the first trees, which emerged around 350 million years ago.