Infants are active learners at birth. They are especially good at using their developing physical skills to interact with the environment in new ways. One powerful way babies learn is by making predictions about how things (and people) work and then testing these ideas. Simple actions like sticking out their tongues at an adult or grabbing, banging, and dropping objects can serve as "experiments" testing to see how people and objects respond. With each test, babies add to their knowledge base–which often encourages a new round of experiments. By creating and testing hypotheses (sometimes over and over again), babies acquire an increasingly accurate set of ideas about how the world works.