“We’ve been making glass for thousands of years, and we still don’t have a good idea of what it is,” says Mathieu Bauchy, a glass expert and materials researcher at UCLA. Most glasses are made by heating and then quickly cooling a mixture of ingredients. In the case of flat glass, which makes up windows, that mixture may include sand (silicon dioxide), lime, and soda. Silicon provides the transparency, calcium provides the strength, and soda reduces the melting point. The swift cooling process doesn’t allow for atoms to form a regular pattern, explains Steve Martin, a glass scientist at Iowa State University.

That helps explain why glass is neither a crystalline solid nor a liquid, but rather an atomically disordered (or amorphous) solid. The atoms within want to reassume a crystal structure, but typically cannot because they are essentially frozen in place. You might have heard that cathedral windows flow over long periods of time, hence why some are thicker at the bottom. That’s false: Such windows were made that way, due to a manufacturing technique that involved spinning molten glass that created uneven patches. But glass does move; it just does so very slowly. A study published last year in the Journal of the American Ceramic Society estimated that room-temperature cathedral glass would take over 1 billion years to flow a single nanometer.

Though natural volcanic glasses like obsidian were fashioned into tools early in human history, glass was probably first manufactured in Mesopotamia more than 4,000 years ago. Likely, it was developed as an offshoot of ceramic-glaze production. The technique soon spread to ancient Egypt, and the first glass objects consisted of beads, amulets, and rods, often colored with added minerals to look like other materials, says Karol Wight, the executive director at the Corning Museum of Glass.

By early in the second millennium B.C., craftsmen began making small vessels like vases. Archaeologists have unearthed cuneiform tablets that spell out the recipe for such materials, but these were written in cryptic language meant to conceal trade secrets, Wight adds.

Glass had already become a serious business by the dawn of the Roman empire. The writer Petronius recounts the tale of a craftsman presenting Emperor Tiberius with a piece of allegedly unbreakable glass. Tiberius asked the craftsman, “Does anyone else know how to blow glass like this?” No, the craftsman replied, thinking he’d made it big. Without warning, Tiberius had the man beheaded. Although Tiberius’s motives remain mysterious, one can imagine such an invention would’ve disrupted Rome’s important glass industry, the first of its kind.

The first big innovation came in the first century B.C., when glassblowing was invented around Jerusalem. Soon the Romans figured out how to make glass relatively clear, and the first glass windows appeared. This was an important shift; previously the material was valued primarily for its color and ornamental properties. Instead of looking at glass, people could now look through it. Within a couple centuries, Romans began producing glass at an industrial scale, and it eventually spread throughout Eurasia.