Watch CNBC's full interview with Wharton's Jeremy Siegel on markets

Jeremy Siegel, professor of finance at the University of Pennsylvania's Wharton School of Business, joins CNBC's 'Squawk Box' to discuss markets, which have been in the red for three weeks in a row. Siegel explains why he thinks the Nasdaq and the S&P 500 could potentially enter bear market territory and how investors can navigate the turbulence. "In the longer run, I still think the S&P has a very good good chance to be up this year by the third and fourth quarter," Siegel tells CNBC.