Bill Gates thought Elon Musk became 'super mean' after finding out Gates shorted Tesla stock

• Elon Musk shot down Bill Gates' attempts to talk about giving to charity after Gates shorted Tesla stock.
• "He was super mean to me, but he's super mean to so many people, so you can't take it too personally," Gates said.

An excerpt of an upcoming biography of Elon Musk shed light on what Musk and Bill Gates thought of each other following a negative exchange over Gates taking a short position against Musk's Tesla.

Walter Isaacson, a writer who spent three years trailing the Tesla CEO, said that in early 2022, Gates was talking to Musk about philanthropy, having proposed that Musk donate to refugees, American education, finding a cure for AIDS, mosquito eradication, and climate-resistant seeds.

Isaacson detailed Gates' and Musk's encounter in his forthcoming biography, "Elon Musk." The book is set to be published on September 12. CNBC ran an excerpt of the biography on Sunday.

After their initial chat, when Gates followed up with Musk again in April via text, Musk shot back: "Sorry. I cannot take your philanthropy on climate seriously when you have a massive short position against Tesla, the company doing the most to solve climate change."

Shorting a stock means an investor is betting on its value to drop. Musk said in April 2022 that Gates had taken a half-billion dollar short position against Tesla.

Speaking about this incident to Isaacson, Gates told the writer he even apologized to Musk.

"Once he heard I'd shorted the stock, he was super mean to me, but he's super mean to so many people, so you can't take it too personally," Gates told Isaacson.

Gates also told Isaacson his decision was a business move, as he anticipated the supply of electric vehicles would soon outpace demand, leading to a price drop.

The full text exchange between Musk and Gates — which Isaacson recounted in the biography — was first leaked online in April 2022. Musk confirmed its authenticity on Twitter.

Explaining how he heard of the short, Musk tweeted: "I heard from multiple people at TED that Gates still had half billion short against Tesla, which is why I asked him, so it's not exactly top secret."

That same month, Musk later took a jab at Gates about his weight on Twitter before eventually declaring that he was "moving on" from the issue.

To put the stakes of their philanthropy discussion in context, Gates and Musk were two of the biggest charitable donors in 2022 by the size of their donations, the Chronicle of Philanthropy reported.

In 2022 alone, Gates contributed $5 billion to the Bill and Melinda Gates Foundation, which was the largest single donation that year. Meanwhile, Musk donated $1.9 billion in Tesla stock in 2022, making him the second largest donor that year, behind Gates and ahead of Michael Bloomberg, the Chronicle reported.

Gates and Musk did not immediately respond to requests for comment from Insider, sent outside regular business hours.