Welcome to Science Buddies, Disneylover101!
LiKe c14er had recommended above, I suggest that you review an overview of the project provided by the link:
http://www.sciencebuddies.org/science-f ... background Your purpose should explain what you are trying to prove in a written statement. Try to make it original and creative!
Specifically, the purpose statement should include the problem you are trying to solve with experimentation, why you want to do the experiment, and your thoughts on the impact your project might have on society.
In your project, the problem you will probably be investigating will be to compare the greasiness of different types of potato chips/crisps.
Then, think about why you became interested in the project. Did you become concerned about excessive fats from potato chips that may be harmful to the body?
Also, you should come up with a few impacts your project might have such as promoting reduced fat potato chips in the market.
I hope this helps and feel free to ask more questions!
-Grace