Question: A newscaster interviewed a senator about a proposed fence along the Mexican border. He said not to build it because some illegal entrants would still manage to climb it or cut through it. Assume for now that the senator is correct, and also assume that there are good reasons to exclude illegal entrants. Does his reasoning lead you to conclude that the fence is a bad idea? Would it be a bad idea if it stopped 95 percent of illegal crossings? 70 percent? 10 percent? How would an economist decide on the critical percentage at which the fence should not be built?