Drain the juice from the pineapple (leave the rings in the can) and bring to a boil.
Combine the boiling juice and the jello and mix really well. Do NOT add the cold water like the jello box directions say.
Pour the mixture into the can of pineapple rings. Run a fork down the center to get the jello in between each ring. Chill. When ready to serve, invert the can on the serving dish, and slice between each ring.