Apple (NASDAQ:AAPL) stock has been on a tear recently, rising more than 20% in the past six months. The stock's sharp rise comes as the tech giant proves it can still increase its iPhone sales. And Apple CEO Tim Cook believes more runway for growth for smartphones will continue to lift the iPhone segment.

In Apple's first fiscal quarter of 2017, the tech giant reported 5% year-over-year growth in both iPhone revenue and unit sales, reversing a three-quarter streak of declining iPhone sales -- a sobering trend for a company that generates over 60% of its revenue from its smartphone business. A return to iPhone growth, therefore, has been welcomed warmly by investors.

Chart showing year-over-year growth in iPhone sales.

Data source: Apple's quarterly operating data. Chart by author.

Of course, despite Apple's recent reversal of iPhone sales, there's still concern that maturing smartphone sales could render the tech giant's return to growth short-lived. But Cook strongly disagrees with bearish sentiment toward iPhones.

"I think the smartphone is still in the early innings of the game," Cook boldly stated in the company's first-quarter earnings call. He went on to explain that there's "lots more to do" as smartphones become more integral to people's lives.

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