My Dd is in 3rd and this year we took regular indoor nails and put them in different types of water - beach water (we live on the coast), rain water, tap water, and distilled water. We left the nails in the water to rust - it was fun and easy - we took pictures along the way to show progress. The difference in "weight" was not enough to weigh with the postal scale we had so we used "visual" changes to determine which rusted more. She got 3rd place in the school.