Pair students up and give them a sheet of aluminum foil, about 10 inches square, along with some paper clips. Wrap up 10 paper clips in a ball of foil and they will see it sink in a tank of water. Ask them to figure out a way to make it float using the same materials. Students will eventually figure out how to make a boat with the foil that will hold the clips. According to the Science Project Ideas for Kids website, this happens because the foil boat has more volume than the foil ball, therefore displacing more water and making the water's upward buoyant forces greater.
Fill a glass with tap water and slowly place an uncooked egg in the water. The egg will sink to the bottom. Remove the egg and stir 3 to 4 tsp. salt into the water until the salt is mostly dissolved. This time when the egg is placed in the water it should float. In "Suspended Egg," Questacon explains that this happens because of density. Salt water has a higher density than fresh water. As an egg is denser than fresh water, but not as dense as salt water, it will float in salt water but sink in fresh water.
Fill a large tank with water and provide students with several cans of soda. Make sure they are all the same size and weight, but provide different brands of regular and diet soda. Instruct students to determine whether any float or sink and come up with some results. According to the Easy Kids Science Experiments website in "Understanding Buoyancy" students should come to the conclusion that all the diet sodas float in the water. This is because regular soda has a high concentration of sugar. Diet soda's artificial sweeteners cause the soda to have a density less than the water, therefore these float.
Many elementary students will enjoy making raisins dance more than they do eating them. When raisins are placed in water, they sink because they are denser than the water. However, place them in a club soda and wait about a minute to see what happens to them. The bubbles from the carbonation in the soda will attach to the raisins, making them more buoyant, and lift them up to the surface of the soda. As the bubbles pop, the raisins sink to the bottom and the process starts again. According to PBS Kids, teachers can explain that this is similar to a child wearing a life-jacket because the jacket makes the child more buoyant and he will be able to float on top of the water.