Add food coloring to water in an ice cube tray, using only one color for each section. Use at least two different colors, but three or four colors would be ideal. Freeze the ice cubes. Place the colored ice cubes into clear plastic cups, combining different colors. Have your students observe what happens as the ice cubes melt. Try as many different combinations as possible to see how many colors you can make.
Fill a bowl with water and then sprinkle some pepper on top of the water. Place your finger in the water to demonstrate that nothing happens. Put a drop of dish soap on your finger and then place your finger in the water again. The pepper will seem to back away from your finger. Explain that the soap is moving the water, not the pepper. The pepper just happens to be floating on top of the water.
Fill a glass about three-quarters with club soda. Drop in a few raisins. The raisins will fall to the bottom, but make sure your students continue to watch. After about 30 seconds, the raisins will start to rise to the top of the soda. When they reach the top, they'll sink back to the bottom. The raisins will continue to "dance" in the glass of club soda because the bubbles stick to the sides of the raisins. This causes the raisins to rise. When they reach the surface, the raisins lose some of the air bubbles and fall back down.
Examine the healthiness of snacks in a way that your students can see. Come prepared with several different types of crackers. Some possible crackers are Ritz, Triscuits, Cheez-Its, Saltines, club and oyster crackers. Place one or two of each type of cracker on a paper towel, preferably the brown ones often found in classrooms. Leave the crackers to sit over night. The next morning, lift the crackers one at a time to show your students the grease marks left behind. Ask them which crackers they think are the healthiest.