I have seen different recipes, but never tried, different variations of this, such as diet soda, flavored soda which gives the cake different flavors, etc. But we just use a cake mix (Read ingredients-MAKE SURE IT IS EGG FREE) and a can of 7up or Sprite mixed together. Put the cake mix in the bowl first and slowly pour in the can of soda. It will foam up pretty high if you pour it in all at once. Then just mix it together with a spoon until combined (no more dry mix to be seen) and pour it into a greased pan and bake according to cake mix instructions...and wallah!
I decided to go in and make a cake really quick after I blogged about this. I will send photos of the finished product when it is baked! :)