Summer Science: Cartesian Divers

One favorite idea I've used at countless science camps and outreach events is Cartesian Divers. (This link is a good source for pipets and nuts for ballast.). A packet of ketchup from a fast food restaurant works, too!  Use cold water and fill a 2-liter bottle to the top.  If you are using pipets and a nut, have a basin of cold water available.  Test the diver in the basin or pail.  If your diver bobs under water as you submerge it, it should work in the bottle.  A 20 oz. soda bottle will work, too and is easier for little hands to squeeze the bottle.