1. Ask your audience the question, "Will this can of regular soda float or sink in the bucket of water?" After gathering everyone's answer, place the can of regular soda in the water and notice that it sinks to the bottom. If the can of regular soda floats, you might have an air bubble trapped under the bottom of the can. 
2. Pick up a can of diet soda and pose the same question. Be sure to point out the fact that the cans are exactly the same size and shape and contain the same amount of liquid (compare the number of milliliters… probably 355 mL). Place the can of diet soda in the water. It floats! Wobble the can from side to side to show your audience that there are no bubbles trapped under the bottom. It still floats. Why?
3. Let your group experiment with different kinds of soda. Why do the diet sodas float and the regular soda cans sink, no matter the brand?

Try the experiment again using salt water. Are your results any different? What if you continue adding salt? How much salt do you have to add before your results change?

Consider changing the temperature of the water or the temperature of the cans. Do either of those changes affect the results?

How does it work?

This demonstration is an excellent way to learn about density. We are all familiar with the basic concepts of sinking and floating. Objects less dense than water float, and those more dense than water sink. Empty cans float, rocks sink. This is only possible because of differences in density.

If both diet and regular soda cans are placed on a double pan balance scale, it would be clear that the regular soda is heavier than the diet soda. This demonstrates the difference between mass and volume. Mass refers to how much stuff exists within an object. If something is heavier than another object, it contains more mass. Mass is measured in grams.

Volume, on the other hand, refers to how much space an object occupies. For fluids, volume is usually measured in liters (L) or milliliters (mL). There are 1000 mL in one liter.  This is what we were referring to when we told you that the cans contained the same amount of liquid - 355 mL. Since both cans have the same volume, the heavier can must have a greater mass. We can now conclude that the heavier can is more dense than the lighter can.

Diet sodas usually contain aspartame, an artificial sweetener, while regular sodas use sugar. Take a look at the nutritional information on the side of the cans. Notice how much sugar is in a regular soda (look under carbohydrates). Most regular sodas have about 41 grams of sugar. How much is 41 grams? Try 18 packets of sugar like the ones you might find at a restaurant! Yikes! That's a lot! Diet soda is flavored with a relatively small amount of an artificial sweetener (like aspartame) which is 200 times sweeter than an equal amount of sugar. Therefore, only a tiny amount of aspartame is needed. Both sugar and aspartame are more dense than water, which can be easily demonstrated by adding small amounts of each to a container of water (they sink). So it is actually a matter of how much of each is used. The 41 grams or so of sugar added to a can of regular soda make it sink. The relatively tiny amount of aspartame used in diet sodas will have a negligible effect on the mass, enabling the can to float.

Why do cans of diet soda float? It is all due to the fact that there is a little bit of space, called "headspace," above the fluid in each can of soda. This space is filled with gas, which is much less dense than the soda itself. It is this space above the soda that lowers the density of diet drinks just enough to make them float. Sugared drinks also have this headspace, but the excessive amount of sugar added makes the can more dense than water.

There is an easy way to calculate the density of the two types of soda to further examine why one floats and the other sinks. Mathematically speaking, the equation for calculating density is Density = Mass divided by Volume. You can calculate the density of both the diet soda and regular soda by dividing the mass (weight) of the soda (in grams) by the volume of the soda (in milliliters).  You will quickly see that the density of the regular soda is greater than the density of the diet soda... so, one sinks and one floats!