What happened

Shares of new initial public offering (IPO) electric car upstart, and purported Tesla-killer Rivian Automotive (RIVN -0.89%) tore up the track yesterday, racing out of the gate on IPO Day to score a 29% gain.

Do you think a gain of that size might have caught a few investors' attention, and enticed them to try to join in the momentum trade? It did. Today as of 10:45 a.m. EST, Rivian stock is up another 10%.

Two smiling kids in a battery powered toy truck.

Image source: Getty Images.

So what

But seriously, that's all that's happening today. There is literally no new news to justify today's rise in stock price. No news to explain why Rivian is now -- as Barron's puts it -- "the most valuable U.S. auto maker after Tesla."

Now what

With a market capitalization rapidly approaching $114 billion, Rivian stock is now more expensive than either General Motors ($89 billion) or Ford Motor Company ($79 billion). Yet over the past year, GM sold more than $130 billion worth of cars and trucks worldwide, Ford sold nearly $135 billion, and Rivian sold... zero.

Now, maybe that makes sense. Maybe Rivian will surprise us and use its order from Amazon for 100,000 electric delivery vans as a springboard to tens and hundreds of billions of dollars of sales in the future. Or -- and bear with me here for just a minute -- this brand new automaker might encounter hiccups as it begins production, and fail to fully live up to the hype.

I'm not saying it will happen. But I am saying it would be prudent to prepare yourself for the possibility that Rivian stock will go back down.