This experiment is very simple and will demonstrate how surface tension causes concave or convex surfaces to form. For this experiment, you will only need two drinking classes, a cork and water. Fill one drinking class with water so that is is almost full. Put the cork in the glass and without touching the sides of the glass, have the class make the cork float to the center of the water. Meanwhile, fill the second class with water all the way to the rim (or slightly above) and place the cork. Have the class record their observations on the position of the cork in both glasses.
Many teachers find that water is the best way to explain buoyancy to students. Make your own iceberg by freezing a container of water. Drop the "iceberg" into a pool of water, and the class will see the ice float. This will demonstrate that ice is lighter than water because the molecules spread out a bit when water freezes. Take the experiment to the next level by adding salt to the water. Add plenty of salt until the water becomes cloudy. Because salt increases the density of the water, the ice will float higher in the water.
Once you have demonstrated the basic principles of buoyancy, your class may wonder how certain large items float. For instance, how does a heavy steel ship float? Or even a concrete ship, such as was used in World War I? To demonstrate that a ship will float as long as it weighs less than the water it displaces, grab a lump of clay and fill a sink with water. Roll the clay into ball and place it in the sink. The class will see that the clay has sunk. Next, shape the clay into a rounded cup so that it fills with air. It should float.
To observe how different objects react in liquid, fill a container with three parts water and one part vinegar. Leave some room at the top so that the liquid can rise without overflowing. Add one teaspoon of baking soda very slowly. When the bubbles settle down, add a second teaspoon of baking soda. Then, add any color of food coloring. Have the students bring in objects from home, such as broken spaghetti noodles, grapes, or other fruit. Drop in the items one at a time and observe which ones float. If the movement slows down, add teaspoons of baking soda as needed.