To clear up this mystery, my business partner and I reached out to psychologists to figure out what kids' rapidly maturing noggins crave. We found that starting around 9 months, babies have the fine motor skills to voluntarily grasp and release. At this age they’ve also started to understand the concept of “in” versus “out”—the idea that objects can be contained inside others. This is why they love to pull tissues out of a box one after another until the container is empty. Once they reach toddlerhood, kiddos become obsessed with water pouring, as that involves many layers of learning: how the liquid feels when it spills, what it looks like as it moves from one cup to another, and the sounds of splashing.