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Blood is approximately twice as thick as water because of the cells and other components found in it.
Human stomach acid is strong enough to dissolve small pieces of metal such as razor blades or staples.
Autoimmune diseases occur when the immune system destroys its own healthy tissues. When this occurs, white blood cells cannot distinguish between pathogens and normal cells.
The longest a person has survived after a heart transplant is 24 years.
It is believed that the Incas used anesthesia. Evidence supports the theory that shamans chewed cocoa leaves and drilled holes into the heads of patients (letting evil spirits escape), spitting into the wounds they made. The mixture of cocaine, saliva, and resin numbed the site enough to allow hours of drilling.