Cartesian diver

A Cartesian diver is a science experiment that shows how air pressure can affect objects underwater. It's made of two bottles, one filled with water and the other partially filled with water. Inside the water-filled bottle is a small object, such as a plastic toy, that can move up and down. When you squeeze the bottle, the air pressure inside the bottle increases and the extra pressure pushes down on the object inside. This makes it sink, just like an actual diver would if they had extra air pushing them down! When you let go, the air pressure decreases and the object inside rises back up. It's like a real diver coming up for air!