Line a fine-mesh sieve with a layer of cheese cloth. To a blender, add the chopped tomatoes and pulse until smooth. Pour the blended tomatoes into the strainer lined with the cheesecloth. Transfer the entire contraption to the fridge and allow to drain for about 8 hours.

Meanwhile, to a blender, add the cubed watermelon, pinch cayenne pepper, salt and sugar. Pulse until the watermelon is smooth. Add the watermelon puree through a strainer and discard the pulp. Pour the tomato water to the watermelon juice and mix. Divide the tomato/watermelon mixture between popsicle molds. Transfer to the freezer for about 1 hour. At the 1-hour mark, push the popsicle sticks into each mold. Freeze until solid, about 6 hours, ideally overnight. To remove, run the molds under warm water and then remove the popsicles.