Now that the United States is using 40 percent of its crop to make biofuel, it is not surprising that tortilla prices have doubled in Guatemala, which imports nearly half of its corn. 

The U.S. currently produces 9.2 billion gallons of ethanol a year and consumes 9.6 billion gallons.

Biofuels are fuels made from recently living organisms. They can be divided into three categories: First-generation biofuels are made largely from edible sugars and starches. Second-generation biofuels are made from nonedible plant materials. Third-generation biofuels are made from algae and other microbes.

Much of the gasoline in the United States is blended with a biofuel—ethanol. This is the same stuff as in alcoholic drinks, except that it's made from corn that has been heavily processed. There are various ways of making biofuels, but they generally use chemical reactions, fermentation, and heat to break down the starches, sugars, and other molecules in plants. The leftover products are then refined to produce a fuel that cars can use.