Jason picked up a plastic water bottle and filled it to the rim with water. He put a ketchup sachet in it and closed it up.

Jason approached his girlfriend Lola and said, "If you can solve how this works, I will give you a dollar."

Jason said, "down" and the sachet of ketchup suddenly went down. Then he said, "up" and it went up. Then he said, "down", and when it was halfway down, he said, "stay" and it stayed.

How did Jason manage to do this?

Assuming there is some amount of air in with the ketchup, (which I question if there is any air at all), Jason could squeeze the sealed bottle to cause the ketchup to sink and release his squeeze for it to float.  But this only works if the sachet has enough air/gas inside.  In this case, squeezing the bottle will increase the pressure inside, thereby compressing the air/gas in the sachet.  The sachet now has a smaller volume, and therefore higher density, and can be made to sink.  judicious control of the squeeze could also keep the sachet indefinity at any position in the bottle

Posted by Kenny M on 2022-03-21 07:33:32