Our rock on a string is a fancy version of something you can easily do yourself. We do it dozens of times a day for weeks at a time, and so have come up with that system. You can do it more simply.
This demonstration comes from a book you might find in the public library called TURNING THE WORLD INSIDE OUT. First you need a fist-sized rock. It will be easiest if it is a little rough so you can tie your strings to it. Then you need some string that is weak enough that the rock almost but not quite breaks it. We use crochet thread, which you can get at a craft store, but sewing thread will work too. Make sure you can easily break the thread without hurting your hand. If there is any chance of hurting your hand, borrow a leather work glove and wear it. Also please make sure the rock falls on something soft and keep us both out of trouble.
Tie two loops (slip knots) in two pieces of string and slip them around the rock so one holds the rock up and the other pulls it down. Now if you pull the bottom one quickly, it should break, if you pull slowly the top one breaks. To be really honest, if the strings are breaking where they are against the rock, instead of on the tail you are pulling, you should tie a stronger piece of string around the rock tightly (duct tape is good.) and tie your weak strings to that.
String breaks when it stretches too far. Because of its inertia (mass), it is easier to move a massive object that is standing still, such as your rock, slowly than it is to move it quickly. When you pull slowly, the rock moves slowly, and your pull and the force of gravity (weight) add together, putting more stretch on the top string. When you pull quickly, the inertia of the rock makes it stay still, and the bottom string snaps before the top one.
I need to warn you that sometimes this experiment misbehaves and the "wrong" string breaks. Actually the right string always breaks, just not always the one you wanted or expected. I once even had both strings break at the same time. But that's physics for you!
I hope you enjoy this project, let me know if you need more information. Good luck!
