The name Pufferfish comes from a puffers ability to inflate its body. To what degree they can do this depends on the species. This ability has evolved as a defence mechanism and a puffer will only inflate if it considers itself in danger. This ability gives them two advantages against escaping a predator.
Some puffers also have an additional line of defence of spines which cover their bodies.
Once puffed up, these spines stick out further giving the predator a nasty shock. This puffing is achievement by drawing water in and expanding the puffers body to make it larger. Becuase of this puffers should never be allowed to inflate out of water as they will draw in air and may never deflate. A more scientific explanation can be found in How Puffers Puff - A Scientific Explanation . In addition to their ability to inflate, puffers have a second defence of being poisonous - the poison being Tetradotoxin .