The lead in a pencil breaks quite easily. The lead in a pencil is weak. Some people try to break the pencil itself. But it is hard to break pencils. This shows that:

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.