Educational toys (sometimes called "instructive toys") are objects of play, generally designed for children, which are expected to stimulate learning. They are often intended to meet an educational purpose such as helping a child develop a particular skill or teaching a child about a particular subject. They often simplify, miniaturize, or model activities and objects used by adults. Although children are constantly interacting with and learning about the world, many of the objects they interact with and learn from are not toys. Toys are generally considered to be specifically built for children's use. A child might play with and learn from a rock or a stick, but it would not be considered an educational toy because 1) it is a natural object, not a designed one, and 2) it has no expected educational purpose. The difference lies in perception or reality of the toy's intention and value. An educational toy is expected to educate. It is expected to instruct, promote intellectuality, emotional or physical development. An educational toy should teach a child about a particular subject or help a child develop a particular skill. More toys are designed with the child's education and development in mind today than ever before. Toys have changed substantially throughout history, as has the concept of childhood itself. Educational toys in particular tend to reflect the cultural concerns of their time. Research on the history of toys and their use tends to focus on western cultures, but work has also been done on North Africa and the Sahara. Dolls can be seen as an early "educational toy" because dolls acted as substitutes, allowing children to learn to care for living babies and children. Similarly, toy bows and arrows and other weapons acted as substitutes for real weapons, enabling children to develop skills needed for hunting or fighting. Up until the 20th century, however, manufactured toys were not readily available, and most often were owned by wealthy families.:174 Nonetheless, "We often forget that throughout history, children have happily played without toys and manufactured playthings."