You can be the most brilliant learner but if you fail to commit new ideas to your long-term memory the effort is wasted.
This is doubly true for babies, whose capacity to remember develops rapidly during the first year. At around eight months (though there is evidence that it's earlier), infants acquire the concept of object permanence. Play peekaboo before this time and your baby believes that you have winked out of existence the moment you cover your face. The proof is that they look away immediately: their attention switches to something else. But once they understand objects persist, that they don't simply disappear, your baby will continue to look at your hands, waiting for you to return. If by some trick you don't, their eyes widen, demonstrating their surprise at the impossible event.