New York (CNN Business) 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.

Tesla TSLA Ordinarily, glass shattering after being hit with a steel ball would not be surprising, butworkers had just gone through a lengthy demonstration dropping steel balls down a long vertical shaft onto a piece of Tesla's new window glass to prove there was no effect. Then, when Tesla 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.

Elon Musk learned something during the Cybertruck's unveiling: he should have hit the windows first, then the doors.

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 ..."

Yup. Sledgehammer impact on door cracked base of glass, which is why steel ball didn't bounce off. Should have done steel ball on window, *then* sledgehammer the door. Next time ... — Elon Musk (@elonmusk) November 25, 2019

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