Subject: How would i measure the amount of rust?

I am doing a scienc fair project on which liquid will rust a nail the most! i 
am using 5 steel nails and i am using rain water, tap water, orange juice, 
coke, and vinegar as my liquids. i will put one nail in each liquid and see 
which nail rusts the most in a peroid of 2-3 weeks. Which liquid do you think 
will rust the most and why? also how should i measure the amount of rust when 
i observating? should i do it in percentage or something? Thanks! 

Re: How would i measure the amount of rust?