“Bring your own blood” might sound like the name of a death metal group (dibs!), but it’s actually the unwritten policy of most cruise lines. Instead of stockpiling and storing blood onboard in the unlikely event that someone needs a transfusion (like this instance aboard a Carnival ship), the ship’s doctors and nurses will instead turn to the few thousand passengers onboard to donate. This, as much as anything else, proves that cruise ships really are floating cities.