(Reuters) - A handful of tweets and four days later, Silicon Valley billionaire Elon Musk has closed orders for his latest novelty product, after selling 20,000 flamethrowers at $500 a piece.

"Guaranteed to liven up any party!" was Musk's tagline for a sale which raised $10 million for his high-speed tunnel venture The Boring Company. bit.ly/2tBqMWb

The man behind electric carmaker Tesla (TSLA.O) and rocket company SpaceX had earlier sold 50,000 Boring Company hats in similar fashion.

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The latest impromptu fundraising scheme comes at a time when financial analysts and investors in Tesla are wondering if Musk will have to seek billions more in funding for the electric carmaker as it struggles to meet promises on production.

"When the zombie apocalypse happens, you'll be glad you bought a flamethrower," he tweeted last Saturday. "Works against hordes of the undead or your money back!"

Elon Musk, founder, CEO and lead designer at SpaceX and co-founder of Tesla, speaks at the SpaceX Hyperloop Pod Competition II in Hawthorne, California U.S., August 27, 2017.
Reuters/Mike Blake

Each flamethrower, he said on Thursday, would come with a free fire extinguisher and buyers need to agree to certain conditions before getting the device in spring.

(Reporting by Supantha Mukherjee in Bengaluru)