By the mid-nineteenth century, technological developments such as the invention of sheet metal stamping machines facilitated the mass production of inexpensive toys, notably tin toys or penny toys. Other technological developments included the advent of papier mache and "India" rubber moulding machines which effectively lowered the costs of manufacturing dolls. To stimulate demand for toys, it was necessary to encourage parents to purchase toys for their children. However, to do that, it was necessary to change attitudes to toys; to promulgate the idea that children were children rather than 'adults in training' and that toys were developmentally useful.