Marrow, Bone Marrow
Your bones have a scooter that moves through the bone marrow inside them.
If you think about it, bone marrow is really just a road for tiny blood scooters.
ずい
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If you put your ear really close to a bone with marrow still in it, you'll hear the scooters going zuwee (ずい) as they move the blood around inside there.
That's how doctors can tell if there's something wrong with your bone marrow. If they can't hear the zuuwee of the scooters, you're gonna die soon. I mean, maybe, I'm not a doctor.