Bring a small pot of water to boil over high heat. Add the tofu and cook for 1 minute. Drain immediately. When the tofu is cool enough to handle, cut it into 3 pieces. In a small, nonreactive bowl, place the fresh water, salt, and lemon juice. Stir well to dissolve the salt. Add the tofu (bowl should have enough water to cover the tofu; add more water if necessary). Cover and marinate tofu overnight in the refrigerator. Tofu feta will keep refrigerated for several days. When ready to use, remove from brine and pat the tofu dry. Cut into small cubes.