Starbucks is reportedly looking at buying Chipotle, a deal that would be good for Chipotle shareholders and a big bet for Starbucks investors.

• Starbucks CEO Brian Niccol, who ran Chipotle before and grew it roughly tenfold, is now eyeing his old company.
• It would be one of the largest restaurant mergers ever, bigger than Burger King buying Tim Hortons.
• The logic: both chains sell a step-up version of fast food, pick similar locations, and chase the same customers.
• Combining the two rewards programs and selling Starbucks coffee alongside breakfast burritos is the obvious payoff.
• Starbucks has been struggling, and is also fighting lawsuits claiming its "sugar-free" drinks contain plenty of sugar.

Outlook: Nothing is confirmed yet, so this remains a rumor — but the market liked the idea on the Starbucks side.