Bring the water to a full boil.
Place the dry flavored gelatin in a heat-safe mixing bowl and pour the boiling water over it.
Stir steadily for 2 full minutes, scraping the bottom and sides of the bowl, until the gelatin is fully dissolved and no granules remain.
Lightly spray a 9x13-inch pan or nagashikan mold with cooking spray, then wipe away the excess with a paper towel so only a thin film remains.
Slowly pour the dissolved gelatin into the prepared pan, aiming toward one corner to reduce splashing and foam.
Poke or remove any bubbles from the surface with the tip of a spoon.
Place the pan on a level refrigerator shelf and chill for 3 to 4 hours, or until the gelatin is completely firm and springs back lightly when touched.
Fill a sink or shallow basin with about 1 inch of warm water and place the bottom of the pan in the water for 10 to 15 seconds.
Remove the pan from the water, dry the bottom, and check whether the gelatin has loosened around the edges.
Cut the gelatin into squares or decorative shapes with a thin knife or small cookie cutters, wiping the blade between cuts as needed.
Serve the Jello Jigglers chilled and keep them refrigerated until close to serving time.