diet coke floats, regular coke doesn't

Why does the diet coke float while the classic coke sinks?

The artificial sweetener used in diet coke is hundreds of times sweeter per unit volume than regular sugar. Classic coke has about 11 teaspoons of sugar in each can which are taken up by water in the diet coke can. Since sugar is more dense than water, it is not surprising that the diet coke will be less dense than classic coke.