You fill the pen with water and just move it over the page. so no mess no drawing on walls or tables and anything else child can get his hands on. Once the water runs out just refill it and it's good to go. The pen does not leak, only takes 1-2 oz of water, doesn’t need refilled often, and stores right in the book cover. Colors of the pages are vibrant and reveal hidden images. Books are easily packed in a bag or carried by a child.