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physics × 291astronomy × 19
history × 18
water × 17
electromagnetism × 17
biology × 13
chemistry × 13
energy × 11
consumer-products × 9
safety × 9
geology × 8
physiology × 7
environment × 7
technology × 7
mobile-phones × 7
weather × 7
medical-science × 6
aviation × 6
radiation × 6
weapons × 6
nuclear-energy × 6
moon × 6
electronics × 6
lightning × 6
alternative-medicine × 5
