Strong, hard, tough. These sound like different ways of saying the same thing, perhaps describing a really good suitcase. But when applied to the physical properties of materials, each of these words has a very specific technical meaning that distinguishes it from the others. And those definitions explain why it’s so difficult to make a bicycle lock that can foil thieves.
Imagine a chain made of diamond. It would be impossible to cut with any hacksaw, but it could easily be defeated with a brick. Diamond is hard, but also brittle, much like glass. What diamond lacks is toughness: the ability to absorb energy without failing.