Help with children's science project please? We want to make an egg bounce...?

We have found that if you submerse an egg in vinegar, that the calcium will be "eaten" away, leaving a rubber like "egg" behind. The problem is (as a Mom), how do we make this an interesting science project? Would the hypothesis have to do with acidity?

Should they try other substances and if yes, what would be good ones for me to have on hand?

I don't want to sound like I am doing this for my twin 8 year olds, and I expect them to carry out all of the experiments and develope their own theories, but the school has indicated that they could receive some outside help. Where could my kids find the necessary information to do a good job?

Thank you!