You will need a bag of M&M's and this form.

It's relative simple.  First review the scientific method with the kids.  Then have them come up with a question to ask about the bag.  My kids wanted to know how many of each color was in the bag.  Next they made a predicition/hypothesis of how many of each color there was.  Then they sorted into different piles and counted.  They recorded the information on the science form and drew conclusions.  They also compared each others results with their own.  They used the results from all their experiments to come up with an average for each color found in the bags.  Of course you have to eat the M&M's when your done :o)  It's an easy, tasty experiment that gets the basic point across.  We followed the form to help us go throught the steps and we talked about how you have to do an experiment several times to get a solid conclusion.