Babies refuse to crawl over a fake cliff, even when it's just glass

Psychologists built a table with a checkerboard pattern under glass — flush on one side, dropped several feet on the other, creating the illusion of a sudden ledge. Most six-to-fourteen-month-old infants refused to crawl onto the 'deep' side even when their mothers called from across it. Newborn animals of many species avoided the drop on their very first exposure, suggesting depth perception works almost as soon as a creature can move.

— Eleanor J. Gibson & Richard D. Walk, The 'Visual Cliff' — Scientific American, Vol. 202, No. 4, 1960