In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: There seems never to have been a civilization without toys, but when and how they developed is unknown. They probably came about just to give children something to do. In the ancient world, as is today, most boys played with some kinds of toys and most girls with another. In societies where social roles are rigidly determined, boys pattern their play after the activities of their fathers and girls are being prepared, even in play, to step into the roles and responsibilities of the adult world. What is remarkable about the history of toys is not so much how they changed over the centuries but how much they have remained the same. The changes have been mostly in terms of craftsmanship, mechanics, and technology. It is the universality of toys with regard to their development in all parts of the world and their persistence to the present that is amazing. In Egypt, America, China, Japan and among the Arctic people, generally the same kinds of toys appeared. Differences depended on local customs and way of life because toys imitate their surrounding. Nearly every civilization had dolls, little weapons, toy soldiers, tiny animals and vehicles. Because toys can be generally regarded as a kind of art form, they have not been subject to technological leaps that characterize(...) inventions for adult use. The progress from the wheel to the cart to the automobile is a direct line of ways up. The progress from a rattle used by a baby in 3,000 BC to one used by an infant today, however, is not characterized by inventiveness. Each rattle is the product of the artistic tastes of the time and subject to the limitations of available materials.
Answer: their social roles are rigidly determined
The reason why the toys most boys play with are different from those that girls play with is that _ .