Elon Musk – CEO of Tesla and SpaceX – is now selling a flamethrower online, and they are moving like hotcakes.

“I want to be clear that a flamethrower is a super terrible idea. Definitely don’t buy one. Unless you like fun,” he wrote on Instagram on Sunday.

The flamethrower is being offered by Musk’s tunnel-digging company, the Boring Company, and pre-orders for the weapon started on Saturday.

Since then, over 10,000 flamethrowers have been sold, Musk said on Twitter. At US$500 a pop, that’s $5 million in revenue – in less than two days. The Boring Company has 20,000 flamethrowers in stock.

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“When the zombie apocalypse happens, you’ll be glad you bought a flamethrower. Works against hordes of the undead or your money back!” he joked on Twitter.

The next day he clarified: “The rumour that I’m secretly creating a zombie apocalypse to generate demand for flamethrowers is completely false.”

The product is also listed as the “world’s safest flamethrower” on the website, with the caveat that fire extinguishers are sold separately.

The U.S. government said “any flamethrower with a flame shorter than 10 ft. is A-OK. [The company’s] design is max fun for least danger,” Musk clarified on Twitter.

Unsurprisingly, the company is also selling a fire extinguisher.

“Buy an overpriced Boring Company fire extinguisher! You can definitely buy one for less elsewhere, but this one comes with a cool sticker and the button is conveniently riiight [sic] above,” the website states.

The company website also sold Boring Company branded hats.