The lime juice keeps the color of the mashed avocado from darkening. Making this appetizer right before serving time, though, is best.
To remove the pit from the avocado, cut the avocado lengthwise in half around the pit, and pull apart the halves. The pit will stay in one of the halves. Firmly and carefully strike the pit with the sharp edge of a knife. While holding the avocado, twist the knife to loosen and remove the pit.