Dissolve the unflavored gelatin in 2 1/2 cups cold water in a bowl.

Combine the fruit-flavored gelatin and hot water in a large saucepan; bring to a boil.

Stir the unflavored gelatin mixture into the boiling mixture; cook and stir until the gelatin is completely dissolved, about 5 minutes.

Remove the saucepan from heat.

Stir 1 cup cold water into the gelatin mixture; pour into a 13x9-inch dish.

Refrigerate until the gelatin is firm, about 3 hours; cut into squares or fun shapes to serve.