Bring 2 cups juice to a boil in a small saucepan over high heat.

Meanwhile, pour water into a large heatproof bowl. Sprinkle gelatin over the water; let stand for 3 minutes. Add the juice and stir to completely dissolve the gelatin, 2 to 3 minutes. Add sugar and stir for 1 minute to dissolve. Whisk in Champagne (or sparkling white grape or apple juice).

Refrigerate until the mixture is the consistency of thin pudding and is just starting to set around the edges, 1 to 1 3/4 hours. The time will vary depending on the size of your bowl and how cold your refrigerator is. After 1 hour, start checking every 5 to 10 minutes because it thickens quickly after that.

Gently but thoroughly whisk the mixture until it's uniform. Ladle just enough into a 6-cup mold to cover the bottom by about 1/2 inch. Nestle 1 cup raspberries into the layer of jello, pushing it into any grooves in the mold. Gently ladle another thin layer of jello over the fruit to help hold it in place. Mix the remaining 1 cup of raspberries into the jello left in the bowl. Gently ladle the mixture into the mold and smooth the top. Cover the mold with its top or with plastic wrap, without touching the jello.

Refrigerate until very firm, about 4 hours. To test if it's ready, touch it with your finger. If your finger does not stick, it's done.

To unmold, moisten fingertips and gently pull the top edges of the jello away from the mold. Fill a bowl with warm (but not hot) water; dip the mold just until the water nears the top. Hold it under for 20 seconds. Take it out of the water. Place a large plate over the mold. Holding the mold and plate together, invert and gently shake to loosen. Then slowly and gently pull the mold off. If the jello doesn't come out, dip the mold back into the water for another 15 to 20 seconds to help loosen it from the sides.