In prehistory, education happened informally through oral communication and imitation. With the rise of ancient civilizations, writing was invented, and the amount of knowledge grew. This caused a shift from informal to formal education. Initially, formal education was mainly available to elites and religious groups. The invention of the printing press in the 15th century made books more widely available. This increased general literacy. Beginning in the 18th and 19th centuries, public education became more important. This development led to the worldwide process of making primary education available to all, free of charge, and compulsory up to a certain age. Today, over 90% of all primary-school-age children worldwide attend primary school