Zest the limes. Then juice the limes. In a small pot, pour in condensed milk and half-and-half. Stir over medium heat until just about to boil. Remove from heat. Stir in salt and allow to cool to room temperature. Drizzle in lime juice, mix well, then taste. Add more lime juice if you prefer it even more tart. Stir in the zest and whisk together until thoroughly combined. Pour into popsicle molds. Freeze for six to eight hours or until completely frozen. Dip in warm water for 10 seconds to unmold each popsicle. Before serving, sprinkle generously each side of the popsicle with crushed graham crackers. Makes 8.