Warren Buffett called into CNBC to surprise 17-year-old Tre Grinner, a Hodgkin's Lymphoma patient who wants to be an investment banker.
With help from the Make-A-Wish Foundation, Grinner has been working as an intern at Goldman Sachs and he was doing a live interview about that experience.
Buffett's main piece of advice for Tre as he prepares to enroll in community college this fall: learn accounting so you can read financial statements. "Accounting is the language of business and there's nothing like getting it early and getting it into your system."
Pointing out that accounting is a "language all of its own," Buffett said, "Getting comfortable in a foreign language takes a little experience, a little study, early on but it pays off big later on."
He also told Grinner to ignore the day-to-day moves of the stock market, like today's plunge on Wall Street, and concentrate on buying stocks in good businesses at good prices. "The lower prices go, as long as you know the company you're investing in, the better it is for a buyer. Down days always make me feel good."
As part of his internship, Grinner was given $50,000 in imaginary money to "buy" stocks. He quickly turned it into $70,000.
Buffett was impressed, saying he wants Grinner to "do that for me."
At the end of the conversation, Buffett invited Grinner to come to Omaha for Berkshire Hathaway's annual meeting in May. "I'd like to meet you personally."
—By CNBC's Alex Crippen. Follow him on Twitter: