Chop a cucumber and puree in a blender. Measure - you need between one and one and a half cups of puree.
Strain puree to separate liquid - reserve pulp.
Sprinkle gelatin over cucumber juice in a small saucepan - let it soften about 10 minutes. Heat gently, stirring, until gelatin dissolves completely. Let cool.
Mix the reserved pulp and the gelatin mixture in a measuring cup. Add yogurt, until you have 2 cups total. Mix.
Pour cucumber mixture into a mold or a bowl to set. Refrigerate at least 4 hours, or overnight.
Unmold and garnish, if desired, for serving.