Toughness is a material's ability to absorb energy before breaking. While graphene is incredibly strong, it is not very tough. This is because graphene is a very brittle material, meaning that it breaks easily when subjected to stress or strain. In contrast, tough materials like rubber are able to absorb a lot of energy before breaking, making them more resistant to damage.

Here's a simple analogy to illustrate the difference between strength and toughness:

- Strength: Imagine a steel rod. It is very strong and can withstand a lot of force without breaking. However, if you bend it too far, it will snap.

- Toughness: Now imagine a rubber band. It is not as strong as the steel rod, but it can be stretched and pulled without breaking. This is because rubber is a tough material that can absorb energy before breaking.

In the case of graphene, its strength comes from its strong carbon-carbon bonds. However, these bonds also make graphene brittle. As a result, graphene is very strong but not very tough.