The Unpredictability of Breakthroughs

Sometimes breakthroughs are caused by working on something else which is being pursued because it's intrinsically interesting. The inventors of the steam engine and penicillin were all working on the thing because they found it interesting or because they were interested in a parochial problem that they didn't predict would blow up into a world-shaking problem. How is it possible that you can try to work on a breakthrough and then actually do it given the growth of knowledge is unpredictable? Well it's also, you know, it's, this only happens occasionally and it's unpredictable in the sense that you can't predict that it will solve the problem you intended either. I'm just thinking of the nice