This is a nice project for the kids on a day when you can't go out and play. This entertained them for hours. But when you're all done, you WILL need to vacuum, and they WILL need to take a bath.
You will need:
a water bottle
a big piece of cardboard
salt or sugar (or sand)
a needle, or drill bit, or awl. (something to poke some holes.)
Step 1: Build your pendulum
Now take the cap, and drill a hole through the top. (about 1/4" diameter)
Step 2: Hang it up
You can tie a string to the end of your 3 stings, to make it long enough. You'll want it about a foot or less above the table top. (or floor)
Step 3: Fill 'er up
Fill the bottle while holding your finger over the hole in the cap. Get ready to remove you finger and give it a good swing. (Not too hard, or you'll shoot salt across the room.)
Make sure the bottle is DRY. If there is any water in there, the salt with clump, and it will not flow out.