Foldable phones like Samsung’s Galaxy Fold and Huawei’s Mate X are coming, whether you’re ready or not. In fact, they’re coming whether they’re ready or not. The software remains untested or nonexistent. The prices are either astronomical or unannounced. But those potential issues can be fixed on the fly. The real thing you should hold out for? Glass.

Yes, glass. The stuff that you tap when you use your smartphone, that you take for granted until you manage to shatter it on the sidewalk. Glass is strong and durable and hard to scratch and easy to see through to the bright and shiny OLED pixels below. Glass can even fold, probably further than you’d think. But it can’t flex far enough for smartphones that open up like books. At least, not yet.

“Glass today, the current choices out there, they’re not optimal” for folding smartphones, says John Bayne, who heads up glass giant Corning’s Gorilla Glass business. “In a glass solution, you’re really challenging the laws of physics, in that to get a very tight bend radius you want to go thinner and thinner, but you also have to be able to survive a drop event and resist damage.”

Instead, the early folding phone manufacturers are leaning on plastic polymers. Which makes sense in that the materials not only can bend as far as you’d need, they can do so repeatedly; Samsung claims its Infinity Flex Display can withstand hundreds of thousands of openings and closings.

READ MORE Get a closer look at Samsung's futuristic, bendy Galaxy Fold smartphone.

“The polymer is better at flexibility; it’s easier to bend at the same thickness,” says John Mauro, a professor of materials science and engineering at Penn State University who had previously spent 18 years at Corning.

But plastic is also, as you may by now have guessed, worse at all kinds of things. It’s much less hard than glass, which makes it easier to scratch and ding up. And unlike glass, plastic will crease over time, leaving you with a large unfolding display, sure, but one bisected with an unsightly wrinkle.

“With the polymer, the molecules can rearrange themselves more easily in response to stress, whereas the glass has a more rigid structure, so the response of the glass is going to be more elastic,” Mauro says. “The structure of the glass will be able to recover after the deformations.”

Don’t take two glass guys’ word for it. “The fact that you're touching [that kind of display] with your nails is scratching it,” Motorola executive Dan Dery told Engadget in a recent interview, about a plastic film cover his company had prototyped. “It has a short life right away; it starts dying the day you unpack it.”

Or just look at how Samsung, Huawei, and others have staged the introduction of their foldable devices. Reporters haven’t been allowed much time to handle them, in part because the software’s not fully baked, but also because the display deviates from the glass cover that the past decade has conditioned smartphone owners to expect. They’ve largely confirmed, though, that the difference is noticeable. It will only become more so over time.

Which means that foldable smartphones won’t be ready until glass is ready. Fortunately, that may be sooner than you’d think. Corning is working on ultrathin, bendable glass that’s 0.1 millimeters thick and can bend to a 5 millimeter radius. The trick, though, is to achieve that kind of pinch without losing the toughness that makes glass great to begin with.

“The back of the problem we’re trying to break, the technical challenge, is, can you keep those tight 3- to 5-millimeter bend radii and also increase the damage resistance of the glass,” Bayne says. “That’s the trajectory we’re on.”