This is an easy-to-do magic (read “science”) trick that will impress any child and a few adults too. All you need is a ketchup sachet, a plastic bottle and water.
Make sure that your sachet floats and put it in a plastic bottle. Then add water so it’s overflowing and close the lid tightly. Now squeeze the bottle hard and make the ketchup sachet sink!
If you want to know why a ketchup sachet sinks, please read explanation below
When the bottle is squeezed, all the pressure is transferred to the ketchup sachet. There is usually a small air bubble trapped inside the ketchup sachet. Unlike liquid, the air can be squashed. The volume of air gets smaller, its density increases and the sachet gets heavier than water. When you release the bottle, air expands back and the ketchup sachet floats again.