Knowledge World

Blood donors in Sweden receive a thank you text when their blood is used.

Mark Twain was born in 1835, a year in which Halley’s comet was visible from Earth. In 1909, he said, “I came in with Halley’s comet…and I expect to go out with it.” When he died on April 21, 1910, the comet was again visible in the night sky.

Army ants that misinterpret the scent trails left by other ants will sometimes break from the crowd and march in circles. If enough ants join, they can form massive "death spirals."