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)