String is very bendy and flexible. But if you pull on string it is very hard to pull apart. You need scissors to cut string. This is because string is:

You know that some materials are hard and others are soft. Some break easily but others do not. Some materials are weak and some are strong. Wool is soft, but if you pull it, it is quite strong. A glass vase is solid. It is hard to the touch. But it can break easily - it is weak if it is dropped.