蔡伦造纸，为什么是划时代发明？

Cai Lun lived in China about two thousand years ago. 蔡伦生活在中国，大约两千年前。 Before paper, people wrote on bamboo slips. 纸出现前，人们把字写在竹简上。 Books were heavy and hard to carry. 那时候的书又重又难搬。 Cai Lun used tree bark and old cloth. 蔡伦用树皮和破布做原料。 He made cheap, light paper. 他造出了便宜又轻便的纸。 Writing and reading became easy for everyone. 从此写字读书方便多了。