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?

(In reply to Solution with other assumptions by Larry)

Ah.  I did not already know about this trick.  More importantly, I did not know that "Jason" was touching the plastic water bottle during the demonstration - so I did not realize that squeezing the bottle, thus changing the pressure in the water bottle's air pocket, was an option.

Posted by Larry on 2022-03-21 13:44:10