Child development is a process every child goes through. This process involves learning and mastering skills like sitting, walking, talking, skipping and tying shoes. Most children learn these skills, called developmental milestones, during predictable time periods. Milestones develop in a sequential fashion. This means that a child will need to develop some skills before he or she can develop other skills. For example, children must first learn to crawl and to pull up to a standing position before they are able to walk. Each milestone that a child acquires builds on the last milestone developed.