When placed on a safe surface that appears to have a dropoff, such as the "visual cliff," 6-month-olds:
◦ refuse to crawl over the cliff even when their parents encourage them to do so.
◦ crawl over the cliff only when encouraged to do so by their parents.
◦ cry and reach out to grasp onto something stable, as if they know they will fall.
◦ crawl over the cliff, but show moderate interest in the perceived depth of it.