Mexican manufacturing has enjoyed a boom under NAFTA. At the same time, U.S. farmers ship oceans of grain to Mexico. Countless products, like those nails, result from manufacturing chains that straddle both countries and ship goods back and forth over the border. American companies profit from the trade - Walmart is Mexico's biggest employer - and that helps to prop up Americans' 401 (k) accounts. American-made parts that are assembled into cars in Mexico and sold back across the border mean fewer jobs in Detroit, but cheaper cars for all Americans.