Add ½ cup fruit juice to a large bowl and sprinkle the gelatin over the top. Use a whisk to briefly mix; it should thicken quickly.
Pour the boiled water into the bowl, and whisk until smooth. There should be no visible chunks, and bubbles will start to form on top. Whisk in the remaining 3 cups of fruit juice.
Prepare an 8-inch square baking dish by lightly spraying it with oil, then press a piece of parchment paper into the bottom of the pan. Pour the jello liquid into the pan and use a slotted spoon to remove as many bubbles from the top as possible. (This is just for looks, so no need to be perfect about it.)
Place the pan on a flat shelf in the fridge to chill until firm, about 4 hours. Once it feels firm to the touch in the center, you can slice it into squares and serve. Leftovers can be stored in an airtight container in the fridge for up to 2 weeks.
Notes
Nutrition information is for 1 of 16 squares. This information is automatically calculated and is just an estimate, not a guarantee.• This recipe works best with sweet juices, like grape juice or apple-cranberry juice. For a sweeter result, you can replace the boiled water with an extra 1/2 cup of juice that has been heated to a boil in a small saucepan. (So you'd use 4 cups juice in total.)• For a firmer jello consistency, reduce the amount of liquid to only 3 cups total. This will create thinner, firmer squares that you can cut out with a cookie cutter to make fun shapes for kids!Update Note: This post was updated in April 2024 with new photos. I also doubled the recipe so you can make it in an 8-inch square pan. To make a half batch, use a 9-by-5-inch loaf pan instead. You can find the original recipe here.