Keeping it chill
The hypothesis that aluminum foil will be able to keep water cold for a longer period of time is proven to be correct.
Aluminum foil helps to maintain the temperature of the food by minimizing the transfer of heat from the food to the surrounding and vice versa. Besides prevented the flow of heat by conduction, the reflective surface of the foil helps to prevent heat loss by radiation - this does not happen for the cling wrap, which does not have an opaque, reflective surface.. Therefore, hot food that is wrapped in aluminum foil will remain hot for a longer period and chilled food that is wrapped in aluminum foil will take a longer time to thaw.
Try to repeat this science fair project using different wrapping materials like bubble wrap or wax paper.
The experiment can also be repeated by using hot water and observing the rate at which hot water cools in containers wrapped with aluminum foil and plastic wrap.
Aluminum foil - http://en.wikipedia.org/wiki/Aluminium_foil
Plastic wrap - http://en.wikipedia.org/wiki/Plastic_wrap
Why should aluminum foil not be used to wrap food that is to be frozen? - http://www.asianonlinerecipes.com/rss/item/1229
Basic safety requirements