Blend or mash the fruit into a puree, or juice it if it is citrus. Straining is optional. If you are using kiwi, pineapple or papaya, boil the puree for 1 to 2 minutes first.
Sprinkle the gelatin evenly over the cold water and let it bloom for 5 to 10 minutes, until it forms a thick paste.
Heat the liquid until very hot but not boiling. A sustained boil weakens gelatin.
Stir the bloomed gelatin into the hot liquid until completely clear, then fold it gently into the fruit puree without whipping in air.
Fill the molds and refrigerate for 3 to 6 hours, ideally overnight.
If they stick, freeze the molds for 10 to 15 minutes right before unmolding.