Babies are born with certain inherent survival mechanisms. One of these prevents them from crawling off of cliffs. How do you test this? Take a baby to the edge of a cliff, let it crawl around, and see if it drops off? Well, you could. A lot of people might take exception to this, though. No. What you do, is take the baby to a room with a partially glass floor, and let it crawl around. If the glass is clean enough, i.e. pretty much invisible, the baby won't crawl on it. You have to be really sneaky when dealing with babies. Another bizarre psychology experiment.