Take a long glass and fill it with club soda containing many bubbles. Cut a raisin in half, drop it into the glass and wait for half a minute. Ask students to watch as the raisin bobs up and down. Draw their attention to the bubbles adhering to the raisin sides. Explain how these bubbles make the raisin float more easily because of buoyancy or flotation. Repeat the experiment using club soda that is flat, without any bubbles, and ask students to note that the raisins don't float. Have students experiment with other items such as popcorn kernels, walnuts and peanuts.
Divide a small amount of clay into two identical pieces. Roll one of the pieces into a ball and mold the other into a boat-like shape. Ask students to observe what happens when you place both objects in a tub of water. The boat-like shape floats whereas the ball sinks. Explain this along the principle of how an object floats if it makes contact with a larger amount of water. Correlate this to how a swimmer needs to keep his body stretched out to float on the water in a pool because curling up may cause him to sink.
Purchase a key chain laser and diffraction grating glasses from a local novelty store. Stand in front of a wall, hold the grating glass in one hand in front of you and with the other, direct the laser beam through the glass. Ask students to observe the pattern produced on the wall. Hold two gratings overlapping each other and gradually rotate them against each other as you pass the laser beam through the overlapping area. The pattern on the wall changes to resemble a molecular grid. Explain that the effects are a result of the diffraction of the laser beam as it passes through the gratings.
Pass the laser beam through a window with a single pane. Ask students to observe and look for two beams -- one that passes through the window and another that is reflected from the window surface. Place a clear tank containing water on a table, sprinkle a little cornstarch into the water and position a microscopic slide inside the tank. Aim the laser beam through the wall of the tank, passing through the slide, and ask students to observe the pass-through and reflected beam.