Lesson: Objects less dense than water float, while object denser than water sink. If you weigh each can of soda, you will see that the Pepsi soda weighs more than the Diet Pepsi. Their masses are different, but their volumes, or the amount of space the fluid takes up, are the same. Diet sodas have artificial sweeteners in it, while regular sodas have sugar. The sugar in the Pepsi can has more mass then the artificial sweetener in the diet soda, which is why it sinks.
In terms of meteorology, airs of different temperatures have different densities. Cold air is denser than warm air. Think of the regular Pepsi soda as cold air and the diet Pepsi as warm air.