How do octopus breathe?

Octopuses "inhale" by sucking water into the mantle cavity (what most of us would call their head). As the water flushes over their gills, blood vessels pull dissolved oxygen from the water and deliver it to the octopus' body. The octopus then "exhales" through its siphon (same tube it uses to expel waste, so it's their asshole too..). When an octopus uses jet propulsion to make a quick escape, it basically takes a huge breath in through the mantle cavities and forcefully exhales through the siphon.