Well I must disagree with Gnomatron on this point (view all node entries for more info), I have seen this experiment done (on video) to demonstrate that babies will indeed crawl off a cliff.
The experiment is set up pretty much as gnomatron describes.
Imagine a one metre by one metre by one metre wooden box (3 feet in imperial measurements), painted in a black and white checked pattern. Immediately next to this is another box with the "floor" painted in the same checked pattern, but with all other sides being made from transparent glass/perspex (capable of supporting the weight of the subject). The glass should be cleaned and lighting of the room appropriate to make the glass as invisible as possible to the subject.
The version of the experiment I saw had the subject placed on top of the checkered box, with a parent of the subject on the other side of the clear box. I.e. the subject would have to crawl across the invisible glass to reach their parent (ie, crawl off the cliff).
The parents were offering as much encouragement as possible to the subject (verbally, visually, tone of voice, offering favourite toys/trinkets etc.)
Here are the results (ages are slightly approximated due to my rusty memory).
A six month old baby will not pause at all, and will crawl straight to their parents.
An 18 month old child will hesitate, but given enough constant encouragement will crawl across the cliff to their parents.
A 3 year old child will generally not crawl across to their parents, no matter the amount of encourangement given. Most will become confused, some will cry, and some will try and find a way off the box (remember, it's 1 metre high, which is a fair distance for a small child).
My only qualm on this experiment is that the vision of extremely young children is impared until a certain age. Unfortunately, I am no expert on this, but I do wonder if this fact has any bearing on the experiment results.
Alternatively, the version of the experiment I saw may have been more an experiment regarding the level of dependancy or trust a baby has in their parents. (Put it this way, if you were standing at the edge of a cliff and God/Jesus/Allah/Buddha/David Hasselhoff told you to "WALK ACROSS TO ME", you'd probably do it.
I am hoping someone with some genuine early-childhood studies qualifications can help to clarify this.