In high school, my physics teacher took two cans of soda from a vending machine, plopped them in an aquarium filled with tap water, and blew our minds. The Diet Coke can floated to the top, while the regular Coke sat on the bottom. Why?
If you like thinking things through, perhaps pause here and noodle on this one.
The answer is a real forehead-smacker: sugar. While both beverages contain mostly water, the regular Coke has a lot of sugar (or dense corn syrup) dissolved in it—roughly 39 grams per can. This makes the density of the Coke liquid greater than that of pure water, so the regular Coke sinks. The can of Diet Coke contains less than one gram of sweetener, resulting in a density nearly equivalent to that of water (indeed, slightly less), so it floats.
If you prefer video explanations, check out this Today I Found Out video, which includes a healthy dose of bonus facts (including plenty on Archimedes and his "eureka" moment!):