5 (.25 ounce) envelopes unflavored gelatin (such as Knox ®)
2 ½ cups cold water
2 (6 ounce) packages fruit-flavored Jell-O® mix
1 cup white sugar
2 cups hot water
1 cup cold water
Dissolve the unflavored gelatin in 2 1/2 cups cold water in a bowl.
Combine the fruit-flavored gelatin, sugar, 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 and sugar are completely dissolved, about 5 minutes.
Remove the saucepan from heat. Stir 1 cup cold water into the gelatin mixture; pour into a 13×9-inch dish.
Refrigerate until the gelatin is firm, about 3 hours; cut into squares to serve.