Twitter's board put up a defense mechanism against Elon Musk. Here's what he could do next

Twitter has indicated that it's not interested in being bought by the world's richest man without a fight. The social network's board made a defensive maneuver that could thwart Elon Musk's takeover bid, but he still has a range of options that suggest this Silicon Valley showdown is likely far from over.

Twitter's board said on Friday it was implementing a shareholder rights plan, known as a "poison pill," that would make it harder and more expensive for Musk (or any other would-be buyer) to acquire the company without the board's approval. The plan came a day after Musk made an offer to acquire all the shares in Twitter (TWTR) he does not own for $54.20 a piece, valuing the company at around $41 billion. That represents a 38% premium over Twitter's closing share price the day before Musk's large ownership stake was revealed.

The poison pill plan, detailed in a filing with the Securities and Exchange Commission on Monday, will remain in place for nearly a year and will be triggered if Musk (or any other investor) expands his stake in the company to 15%; he currently owns around 9% of shares. It would give all other shareholders the right to buy one additional share for each share they own at a discount. While other shareholders executing their rights to buy new stock under the plan would have to pay $210 for each new share they purchased, Musk (or another hostile investor) would have to pay $420. (Twitter's team proved Musk isn't the only one capable of spicing up a serious corporate standoff with marijuana references.)