Critical periods are specific time frames in development when an organism is particularly sensitive to certain environmental stimuli. During these windows, the brain is more receptive to learning and acquiring skills, such as language or social behaviors. If the necessary experiences are not encountered during these times, it may be much more difficult or even impossible to develop those skills later. In humans, critical periods are most evident in early childhood, especially for language acquisition. For example, children exposed to language during these formative years can learn to speak and understand it more easily than those who start learning later. This concept highlights the importance of early experiences in shaping development.