In a study published in Nature Medicine, researchers have trained artificial intelligence on tens of thousands of tissue images to reveal that our organs age at different rates — and that these divergences leave measurable traces in the blood. The work suggests that biological age, organ by organ, may be a more truthful account of a body's condition than the years on a birth certificate. If prospective studies confirm that these blood signatures precede illness, medicine may one day hold a mirror to the body's interior not through surgery or imaging, but through a single vial of blood.