Read More: Elon Musk Considering Tender Offer for Twitter, Says $46.5 Billion Lined Up

Musk currently has about $3 billion in cash or other somewhat liquid assets after spending $2.6 billion buying a 9.1% stake in Twitter in recent months, according to Bloomberg estimates. His other holdings are tied up in Tesla stock — about $184 billion worth — and his ownership of closely held Space Exploration Technologies Corp.

The big question is whether Musk would consider selling part of his stake in one of his prized companies to acquire Twitter.

Musk would need to pledge about 58.7 million Tesla shares to secure the $12.5 billion margin loan facility included in the debt financing. That would bring the total percentage of shares he’s pledged to about 85% of his holdings. The margin loan facility documents do allow Musk to sell his unpledged Tesla shares.

The unpledged shares are worth more than $25 billion, so if he sold almost all of them, it’d be enough to cover the $21 billion in equity financing, after taxes, coupled with the cash he has now.

Alternately, Musk could find partners to contribute to his equity financing. But one thing’s for sure: The mystery over how Musk will fund his bid was only partially solved today.

Musk and Jared Birchall, the managing director of Musk’s family office, didn’t immediately respond to a request for comment on how Musk plans to finance the $21 billion. Shares of Tesla pared early gains of as much as 12%, trading up 8.2% to $1,057.14 as of 10:24 a.m. in New York.

–With assistance from Dana Hull and Shannon D. Harrington.