Add agar agar powder or flakes. Stir and continue to simmer for 2 minutes. If using flakes, you may need to simmer for a bit longer to ensure the flakes have completely dissolved.
Pour into a shallow bowl or dish and refrigerate 30 minutes.
Use any juice you'd like, although agar agar doesn't set well with high acids, like lemon juice.To make the purple "gelatin," steep 2 tablespoons of dried butterfly pea flowers in 4 cups of apple juice until the desired color is achieved. Strain juice and discard flowers before bringing to a simmer and adding agar agar.