Your child will benefit from learning about water pressure and density with the Playsteam Deep Diving Seahorse set. In the booklet included, it shows you how with a difference of water pressure in the water cavity than the water outside in the bottle you put the seahorse in. Any water bottle can be used and is reusable! The booklet explains the physics of why this happens in a constructive way so that your child will understand it. It is a great device for you and your child to connect over an interesting topic that will help them when they get into the world of physics in school. Every Playsteam comes with a comprehensive booklet to explain what the toy does and how it will help your child's educational future. Included is a sticker that can be added to the back of the water bottle of choice and some sea creatures for added effect.