That is an excellent question. I wish the answer were simple.
If you take practical examples, knives are meant to be sharp. They're meant to cut things. They are a consumer product. We wouldn't, by definition, say that they're sharp; they're therefore dangerous.
We have to take a look at the intended use of that product and the amount of documentation that supports it so that the consumer, when using it, has the information they need in order to make sure that they're using it as was intended and that it is reasonable.
There is, in fact, existing jurisprudence about this particular issue as we move forward on how we will interpret what is an unreasonable risk.