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The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
Adolescents often feel clumsy during puberty because during this time of development, their hands and feet grow faster than their arms and legs do. The body is therefore out of proportion. One out of five adolescents actually experiences growing pains during this period.
More than 2,500 barbiturates have been synthesized. At the height of their popularity, about 50 were marketed for human use.
Though the United States has largely rejected the metric system, it is used for currency, as in 100 pennies = 1 dollar. Previously, the British currency system was used, with measurements such as 12 pence to the shilling, and 20 shillings to the pound.