Diet Coke and Mentos Geyser

• Popularized by Youtube videos of 10-foot-tall soda bottle geysers, this popular experiment is easy to set up--if harder to clean up after. Simply place several Mentos candies into a bottle of Diet Coke. The more candies you put in at once, the bigger the geyser will be, so you may want to consider adding a "feed tube" to get as many candies in as possible. Naturally, this should be done outdoors wherever possible.
  
  

Dry Ice Experiments

• Dry ice is a solidified form of carbon dioxide. It turns into a gas quickly when dunked into water, a process known as "sublimation." The process releases a great deal of carbon dioxide "steam" but is harmless. Dry ice is often sold in pellets for the purpose of food processing.
  
  

Elephant's Toothpaste

• You may have heard of this one as a demonstration, but it works just as well as an experiment with a few substitutions. The name of this experiment comes from the large amount of foam it produces. You can produce it by placing dish detergent and yeast into a bottle of 8 percent hydrogen peroxide. The yeast causes the hydrogen peroxide to disassociate into water and oxygen, while the detergent ensures that the bubbles are small enough to create "foam."
  
  

Ninhydrin Forensics

• Frequently used by forensics experts to identify fingerprints, ninhydrin is a relatively safe chemical with a dramatic effect on amino acids, forming a purple substance that is easy to identify. To demonstrate the effects, mix up a ninhydrin solution--the chemical is sold in powdered form--and drop onto a piece of paper that someone has left fingerprints on. The fingerprints will turn purple from the ninhydrin exposure.
  
  Kids are advised to wear gloves and smocks when handling ninhydrin; although the solution is harmless, it has the same staining effect on hands and clothes that it does on fingerprints.