In a medium bowl or large (4-cup) measuring cup with a spout, heat 1 cup Coca-Cola until boiling or nearly boiling.
3 cups Coca-Cola
Sprinkle 6 ounces cherry jello mix into the hot liquid and whisk until completely dissolved. The mixture may bubble a little.
6 ounces cherry jello mix
Stir in the remaining 2 cups of cold Coke and 1 cup rum
1 cup rum
Pour into plastic shot glasses.
Transfer jello shots to the refrigerator and chill for 4 hours until set.
Just before serving, top each jello shot with a Maraschino cherries
Maraschino cherries
Notes
Use up to 1½ cups of alcohol for a stronger jello shot. For a lighter jello shot, you can use additional cold water or Coke and less alcohol. You need four cups total of liquid to combine with the jello mixture.
To make a layered look, combine half of the package of jello with one cup of boiling water. Stir until dissolved. Mix in a half cup of rum and half cup of cold water. Divide among the jello shot cups and chill until set. Mix the remaining jello mixture with one cup of hot Coke until dissolved. Stir in a half cup of rum and a half cup more of Coke. Bring to room temperature. Pour over the set cherry jello layers. Chill until set.
The jello shots may firm up faster than four hours. If you are using shallower shot glasses/cups and if you use icy cold liquid, they will not take as long to set.
Keep jello shots refrigerated until consuming. They will start to liquefy in the summer heat.