In a tweet over the weekend, Elon Musk explained why the supposedly unbreakable windows on Tesla’s new Cybertruck cracked during a demonstration Thursday night. Ordinarily, glass shattering after being hit with a steel ball would not be surprising, but Tesla\n \n (TSLA) workers had just gone through a lengthy demonstration dropping steel balls down a long vertical shaft onto a piece of Tesla\n \n (TSLA)’s new window glass to prove there was no effect. Then, when Tesla\n \n (TSLA) designer Franz von Holzhausen came on stage and threw a ball at the driver’s side window of the truck on stage, a spiderweb of cracks appeared. When von Holzhausen threw the ball at the rear side window, the same thing happened. Musk spent the rest of the presentation standing in front of the Cybertruck with its two smashed windows. On Sunday night, Musk offered an explanation. Shortly before von Holzhausen threw the steel ball at the windows, he had hit the truck’s door with a big sledgehammer. That was to demonstrate the strength of the truck’s body panels. But the impact from the blows of the sledgehammer had also slightly damaged the glass, Musk said. “Sledgehammer impact on door cracked base of glass, which is why steel ball didn’t bounce off,” Musk tweeted. “Should have done steel ball on window, *then* sledgehammer the door. Next time …” And, Musk also tweeted, that extra-strong steel body explains something else about the Cybertruck: Why it looks so weird. The body is made from thick cold-rolled stainless steel of the sort used to build SpaceX rockets, Musk said. (Musk is also CEO of SpaceX.) It’s not the ordinary thin, pliant steel car bodies are typically made from. Auto body panels are usually made using stamping machines that can press the metal into complex sculptural shapes. But you can’t stamp the Cybertruck’s steel, Musk said. “Reason Cybertruck is so planar is that you can’t stamp ultra-hard 30X steel, because it breaks the stamping press,” he tweeted. He continued on in another tweet, “Even bending it requires a deep score on inside of bend, which is how the prototype was made.” That resulted in the truck’s strange flat-edged appearance, which has inspired a number of creative comparisons online. The Cybertruck has been compared to everything from a doorstop or an old Apple Mouse to the SpongeBob Squarepants character Flats the Flounder, or a triangle on wheels.