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The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
Blood in the urine can be a sign of a kidney stone, glomerulonephritis, or other kidney problems.
In 1844, Charles Goodyear obtained the first patent for a rubber condom.
Although puberty usually occurs in the early teenage years, the world's youngest parents were two Chinese children who had their first baby when they were 8 and 9 years of age.
Although the Roman numeral for the number 4 has always been taught to have been "IV," according to historians, the ancient Romans probably used "IIII" most of the time. This is partially backed up by the fact that early grandfather clocks displayed IIII for the number 4 instead of IV. Early clockmakers apparently thought that the IIII balanced out the VIII (used for the number 8) on the clock face and that it just looked better.