|MadSci Network: Chemistry|
Sorry it took so long for us to answer your question, and I hope this is in time for your science fair...
Salt (Sodium Chloride, NaCl) helps rusting of iron because it accelerates the rusting process.
Rusting, also known as "Oxidation of Iron" is the chemical reaction when metallic iron (Fe) reacts with water (H2O), Carbon Dioxide (CO2), and Oxygen (O2) in the air to form Iron Oxide (Fe2O3, FeO, Fe3O4). In order for this reaction to occur, electrons must be transfered from the iron to another molecule, allowing the Fe to become an ion, either Fe2+ or Fe3+. If you add Sodium Chloride to the water, the NaCl breaks up into Na+ and Cl- ions, which help carry electrons back and forth. This speeds up the rusting (oxidation) process.
This really affects me here in Buffalo, New York, because they use salt to melt the snow on the roads (Another cool science fact is why salt melts ice...) and that makes cars rust faster around here.
There's some more stuff about chemical oxidation reactions at Encyclopedia Brittanica Online.
Hope this helps!
Try the links in the MadSci Library for more information on Chemistry.