It's sometimes called "Einstein's Riddle" because, according to legend, Einstein invented it as a child. Others say that the puzzle was actually designed by Lewis Carroll, best known as the author of Alice's Adventures in Wonderland. (Carroll was also a logician.) Where did this brain teaser originate? We're not really sure. Perhaps it was a 1962 issue of Life International magazine.
In any event, "Einstein's Riddle" is a good test of your mental agility. They say that only 2% of the population can solve the problem. The TED-Ed video above will walk you through one version of the riddle. If you don't want any assistance, you can find other versions online.
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