A man 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.
The man approached a woman and said, "If you can solve how this works, I'll give you a dollar."
The man 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 he do this?