I was unaware that PET bottles do not float when 100% filled with water. I assumed they would float, although I did not look-up the density of PET plastic. Not having to buy glass stirring rods means I will lose none in shipment.
Just looked it up: PET has a density of 1.38 grams/cubic centimeter. That's 38% greater than water at 1.00 grams/cubic centimeter. I had no idea. I figgered because it's a thinnish plastic, that a beer bottle made of it would float. I have learned something new, today. And saved a little bit of money...