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Ask the expert
A Brief History Of Measurements
Who defines a meter's length and a kilogram's weight, and how were these units established?
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Brainwash - Where Is The Arctic Circle?
Where would you find Obsidian? What is Streptococcus? A Challenging Scientific Trivia
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Science Panorama
The Time-Travel Paradoxes
What happens if a time traveler kills his or her grandfather? How do you stop a time machine from just appearing somewhere in space, millions of kilometers from home?
