[Learning is Fun] Density
Density = mass /volume
Diet cola is less dense than regular cola. If you took a diet Pepsi and a regular Pepsi and placed them in a bucket of water, the regular Pepsi would sink, while the diet Pepsi would float. The reason is because the regular Pepsi is more dense than water -probably due to the fact that the manufacturer packed 41g of sugar in to a 355ml can. (around 7 teaspoons of sugar equivalent)
Here's another learning fun bit about density. Saturn is less dense than water. So if you had a scoop of Saturn, it would float in your bathtub.