There's a psychological term for that -- visual cliff -- and they test babies (and other baby animals) for it by setting up a glass-topped table so it's as though the table ends. Below a certain threshold of brain development, babies will crawl over the edge with abandon.

A couple years ago I walked over a big grate -- the kind you see in parking lots -- only to find out when I looked down: somebody had left a light on in the shaft about 50 feet down.

I may even have yelled out loud. It was a very sturdy grate that trucks drove over all the time, so it wasn't any kind of dangerous. I leaped to safety and stood there for a while with my heart hammering.

Whereas I'm all, "Dude, that is cool."

Fear of exposure can be unlearned, you know. Not that anyone other than a climber or a professional tree-cutter or linesman would want to, I suppose. But if I stop climbing for a while and then go back, I have to retrain myself so that I don't freak out when I'm up really high on an overhang.

The Mackinac Bridge (http://www.mackinacbridge.org/), a five-mile long bridge that connects the two peninsulas of Michigan, has grating instead of pavement on the two inner lanes. When I was a kid and felt the car drive over it, the grate was just a noisy, vibrating annoyance. Then I went under the bridge on a boat and looked up. You can see the cars driving over the grate--and from the water (or a few feet above it) it doesn't look like they're driving over much of anything that would hold up a car. I've been a fan of the outer lanes ever since.

I think if I'd worked my way onto it voluntarily, I wouldn't have spooked like that, but because of the semi-dark twilight, it wasn't until I was several steps onto it that I looked down.

OTOH, I've been totally unable to walk out on (very short and safe) RR bridges, stepping on the ties over the gaps.

Note to self: brains weird.

My heart was beating like a triphammer in that little enclosed overhang walkway at The House on the Rock, and that had an opaque floor and visible beams/support structure. Not my cup of tea.

Though the big overlook at Lookout Mountain didn't bother me, probably because of the solid rock underfoot.

Man, I would love to walk out into that thing. I want to do the Grand Canyon Skywalk too. I get scared but I can make myself walk over it/out into it, and once I'm there it's cool. I'm fine as long as I'm upright and my feet are on something solid.

Richard Branson planning a glass-bottomed plane for Virgin.

AhahahahahahaNO.

A few years ago, my family went on a cruise, and in the casino, which we had to walk through a lot, the floor had a few of these deep pits filled with various gambling-related stuff, with glass over the top. If you looked down, you saw a few feet of empty space before the stuff began. Every single time I walked over one of these things, I'd stumble a bit. I have no fear of heights at all, but the lack of floor where there had been floor a second ago just confused my brain.

Kate, I'm glad you had a goodish ER visit. Also, I'm definitely glad that Rose is responding to the medications.

Okay. Going to make PJs now.

In the Acropolis Museum in Athens, there's an ongoing archeology work site below the ground floor. Some parts of the floor are clear so you can see the work going on below. It's pretty nifty, and a great way to show the way the museum contents are tied into a living study. [link]

That said, I walked on the solid parts of the floor when I visited.