The reason that octopuses have 9 brains is that the 9 brains are used to control different parts of the body which is strange because humans have one brain and it controls all parts of the body.
But octopuses need 9 brains to control the different parts of their body.
One brain is the central brain that controls the nervous system and the rest of the 8 brains are in each of the octopuses arms.
Those 8 other brains in the octopuses arms control a cluster of cells in the octopuses arms which causes and controls movement of the octopuses arms.
The octopus also has 3 hearts.
The reason that octopuses have 3 hearts is because one heart pumps the blood around the octopuses body and the other two hearts pump blood around the octopuses gills.