Pour the fruit juice or water into a mixing bowl and sprinkle the unflavored gelatin evenly over the surface. Let it sit for 5 minutes until the gelatin grates and blooms, absorbing the liquid.
In a saucepan, bring the boiling water to a simmer and then pour it over the bloomed gelatin. Whisk vigorously until all the gelatin is completely dissolved and the mixture is smooth, with no lumps.
Add the flavored gelatin or fruit juice to the dissolved gelatin mixture, stirring well to combine. This gives your jelly its vibrant color and flavor.
Stir in cold water to help cool the mixture slightly, then pour the gelatin into a clean, brain-shaped mold or silicone mold. Fill it almost to the top with a steady pour to avoid air bubbles.
Place the mold in the refrigerator and chill for at least 4 hours or until fully set and firm to the touch.
Once set, gently unmold the gelatin by inverting the mold onto a serving plate. Slightly wiggle the mold to release the brain shape, revealing the textured, bumpy surface.
If desired, decorate with edible glitter or add small touches of food coloring to enhance the creepy, brain-like appearance before serving.
