Tom Friedman's Open Black Box (2006) is eight corners made of black construction paper, dangling from fishing line. That's it. The eight corners, each with three prongs, have to be hung just right, or their "black box" of 137 by 137 by 137 inches of empty air does not form. Each paper piece is completely free-hanging, with no points of connection between them.

The installation took a day and a half of straight work by Friedman and Eric Adami on the exhibition prep crew at the Henry. Tools: a lift (here's Friedman up on it), a laser plumb, and a pre-made map that gets laid on the floor. The map looks like a drawing of a constellation. Friedman drew the map after he got the cube installed right for the very first time in 2006, using the laser plumb (pointing lines straight down from where each line attached to each corner). Now, he sets down the map and shoots those points up to the ceiling and marking them up top.

But that's still only ballparking it.

At the Henry, the ceiling is far taller than where the piece has been installed in the past. This gives the experience an entirely different cast. Twelve-plus feet of additional fishing line had to be spliced to each existing fishing line at the Henry.

Each of the three prongs on each of the eight corners has a piece of fishing line attached to it, so each tripod is adjustable at three points. The high pieces go up first. Next comes hours of Ummmm, up and to the left...no...down a little...no...yeah...

"He started dropping sculptures from the ceiling, and he would hold it, then ask me if it looked like it was working in alignment with the other units," Adami said. "The first unit is easy to set up, but from then on, they build on the total. From one angle, it will look okay, but then you walk 30 feet away, and you realize that maybe one of the cables needs to be tensioned up a little to be brought into alignment with a different piece, which might then affect another piece... So the more pieces you got up in the air, the more troubleshooting you had to do with each piece."

All of this work is designed to make you mentally materialize something that's not physically there.

On the other end of the Henry's galleries is another mobile and another type of illusion. Fire in the Belly looks ethereal, like a fleeting moment in time. Curved shapes hang in midair like drops of dew from some other universe, or a stroke of paint made by a giant alien. They're a slightly metallic green—it's automobile paint—which gives an extra-soft-and-dreamy appearance. Each segment hangs from fishing line.

But what you can't tell from looking is that this is the opposite of Friedman's light paper, which with its crisp edges, you're sure is solid wood. Shettar's pieces look light as bubbles. But they're made of acacia wood.

"Some of them are, like, 30 pounds," said Dan Gurney, the prep guy who worked on this one, which took a full day to hang, also using a laser plumb and a map and 120-pound-test fishing line. "You'd knock yourself out if you ran into one."

In both cases, but in very different ways, what you see is not what you see, and it changes according to what you know. Since they're mental sculptures as well as physical ones, I just thought you might want to know a little more.