One option for a science project is to test to see what materials cause a nail to rust quickly. For example, observations of a nail over time in a variety of liquids makes for a good basis for a science experiment.
Required materials include some simple steel nails, jars, a pair of tongs and a variety of liquids, such as water, salt water, orange juice, soda and vegetable oil. Simply fill each of five jars with a different liquid and place the nails inside.
Place the jars in a safe place, and have the child check the nails every day at a specified time. The child then notes any observations about the nail, including the amount of rust on the nail. After a week, a significant amount of deterioration should be noticeable on the nails.
A modified and quicker version of the experiment involves using water, salt water and vinegar. Place a nail in each of those substances for approximately 10 minutes, and then pull them out to let them sit for another 10 minutes without wiping off the nails. The child can then observe the differences among the three nails, noting any differences. This can lead to a discussion of oxidation and what causes it to occur.