Three hearts, blue blood, and nine brains

The aha, in writing

1. An octopus has three hearts: one pumps blood around the whole body, and the other two push it past the gills to pick up oxygen.
2. Its blood is blue because its oxygen carrier, hemocyanin, uses copper where yours uses iron.
3. Of its roughly 500 million neurons, about two thirds sit in the arms, so nine brains is a nickname for one central brain plus eight busy arms.