Answer:
Sharks skeleton is made of cartilage instead of bones to have a lighter weight which allows them to go up and down quickly in the ocean without a lot of effort to stay afloat.
Cartilage is robust but not as dense as bone which gives them a great flexibility that allows them bending much easier than bony fishes. Such elasticity also gives them more speed as tails can move faster to propel them through the water.
Links to other pages in this site
Why Are People Afraid Of Sharks?
What Color Is The Hammerhead Shark?
How Many Species Of Sharks?
Why Do The Great White Sharks Attack Humans?
Why Is Mako Shark Endangered?