Cut watermelon into small chunks (without rinds or seeds). Blend them in a blender until you've obtained around two cups of juice/puree.
Soften (bloom) the gelatin by adding it to a bowl with a ¼-½ cup of watermelon juice/puree.
Meanwhile, in a saucepan, heat around ½ cup of the watermelon puree over low to medium heat. As it starts to get warm, add in the softened gelatin and continue to cook and stir until the gelatin is dissolved.
Remove the saucepan from the heat, and stir in the remaining watermelon puree to help cool the mixture.
Pour into molds and freeze. If the gelatin mixture is still pretty warm after adding the remaining cold watermelon puree, allow it to cool to room temperature before placing the molds into the freezer.
If you want an easy way to hold up the popsicles sticks in the molds, you can first gel the gelatin in the fridge. That allows you to place your sticks right where you want them! The gelatin will hold them in the right place while freezing.
Another option is to cut pieces of watermelon rinds to the size of the top of the molds and pierce them with the popsicle sticks. The rind pieces will hold the sticks in place and will make cute-looking popsicles. (If using round molds, you can use lime slices instead.)
Optionally pour a dash of coconut milk on top of the watermelon gelatin before adding the lime slices or watermelon rinds to get the look of the white flesh of the watermelon along the rind. (If you allow the watermelon puree to gel first, you'll end up with layers rather than an ombre look.)
Freeze the popsicles for several hours until frozen solid.
Remove from the molds and serve. If you are having a hard time getting the gelatin popsicles out of the molds, you can run them under warm running water for a few seconds before pulling them out.
Other fruits can also be used to make gelatin popsicles, but avoid fruits with proteolytic enzymes such as pineapple or papaya. If using those fruits, you'll need to use pasteurized juices or will need to cook them for longer to deactivate the enzymes so that the gelatin will gel.Optionally add some cream to the fruit and gelatin mixture to make creamier popsicles.You can also add some sweetener for sweeter popsicles. (I didn't feel that the watermelon popsicles needed any further sweetening!)