Tesla’s stock price plunged 14% on Thursday after the car company reported negative free cash flow of $1.1 billion despite revenue rising 26% from a year earlier. CEO Elon Musk sounded rather odd on the earnings call Wednesday evening, especially when it came to Optimus, his humanoid robot. Gone was the bluster and cockiness of a man who’s claimed there will be 10 billion robots by 2040. Everything that came out of his mouth sounded like a warning that Optimus might take considerably more time than he first promised. “Optimus, as you’ve heard me say before, I think Optimus will be the biggest product ever. But it is a very complex problem to solve,” Musk said on the call. The Tesla CEO went on and on about how difficult it is to design a robot, including considerations like wear and tear, achieving dexterity with the robot’s hands, and making sure it doesn’t break down. Musk also stressed that it would be difficult to produce the robots. “I really want to emphasize here that the production scaling challenge is very substantial,” said Musk. “This is going to be the hardest product to scale manufacturing that we’ve ever made at Tesla because everything on the robot is new. And the difficulty of scaling the production ramp is proportionate to the newness of the parts.”