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What do ETFs invest in?

The first ETFs tracked plain-vanilla indexes, like the S&P 500. But as investors poured money into ETFs, investment companies eager to capture that business began engineering new, supposedly innovative ETFs that are complicated, risky and more expensive - just the reverse of what ETFs are supposed to be. Some of these new ETFs use borrowed money to boost returns. Others track narrow slices of the market, such as nanotechnology.