1.   But unlike Europeans and Asians, who are separated from their homelands by an ocean, Mexican immigrants can look over the invisible border for inspiration.

2.   The last time Richard Matienzo tried, nine months ago in Ponce, Puerto Rico, he could not make his legs carry him across the invisible border.

3.   Across the invisible border, minarets pierce the horizon, and brick farmhouses are surrounded by walls, protecting Muslim women from prying eyes.

4.   But with Israel and the West Bank separated only by invisible borders, with Jewish settlements peppered throughout Arab lands, conflict is always possible.

5.   India, Pakistan fight over an invisible border.