Early suitcases were made of wood or other heavy materials. With the popularity of air travel, the materials of suitcases tend to be lighter hard plastic or cloth. Most models also come with built-in small wheels, which can be easily pulled with a handle. After 2010, suitcases with 4 small wheels also appeared. In addition to being able to rotate 360 ​​degrees, they can be easily pushed on flat ground without any effort. Suitcases can also be divided into hard-shell suitcases and soft-shell suitcases based on the outer shell. Hard-shell suitcases are more resistant to falling and are suitable for luggage that needs more protection. Soft-shell suitcases are usually lighter than hard-shell suitcases and the material is more flexible. However, the zipper of a soft-shell suitcase is more likely to burst due to too many items inside.
