Put the gelatin in the water. Peel the melon and make it purée. Put the melon purée and sugar in a pan and heat up to 80C. Add the well melted gelatin to the melon purée and mix well. Then add lemon juice and mix well. Keep some melon for garnish. Cool off the mix to about 25C with ice water. Add the yogurt to the purée and mix well. Then put it in cups and keep them in refrigerator for a few hours.