Bob has a secret. His secret is not just something he finds valuable, but something lots of other people would find valuable too. Perhaps it's a new way to make energy, or to dye wool or to build yachts. Perhaps it's just his mother's maiden name. We don't know, because it is Bob's secret.
Does Bob have an obligation to share his secret with us? Can we go to Bob's house and demand he tell us his secret? Can we threaten Bob to get the secret out of him, or promise him payment for his secret and then renege? If not, why not?
Alice would like to know Bob's secret, so she offers him a significant amount of money for Bob to tell her. Bob is worried that Alice will tell others, but she promises not to tell anyone else. Having heard Bob's secret, Alice wishes she had thought of it herself and would prefer no-one else to know the secret, not even Bob. This, of course, is impossible, but Bob offers the next best thing: he'll promise to never tell anyone else the secret for a small fee each month. Alice agrees.
This continues for many years until one day Alice meets Claude. Alice very much wants to tell Claude the secret, but she still has an agreement with Bob promising not to tell anyone else. She talks to Bob and he agrees to let Alice out of her promise, for a small increase in his monthly fee. Alice tells Claude the secret, after swearing him to secrecy.
Shortly after having heard the secret, Claude loses interest in Alice. He floats around the world for a while, visiting various places and eventually meets Desmond, to whom he quickly tells the secret. Claude did not swear Desmond to secrecy. In fact, he didn't even tell Desmond that what was being told was a secret. The idea is no longer a secret. Desmond tells everyone.
The idea sweeps the world and everyone talks about Desmond, the man who told the world. Some people think Desmond actually made up the idea himself, but most people believe Desmond when he says a man named Claude told him the idea. Alice and Bob certainly believe it. Alice is upset because Claude broke her confidence. Bob is upset because Alice doesn't want to continue his monthly payments. They go looking for Claude.
Is Alice right or wrong to be upset with Claude? Is Alice right or wrong to stop paying Bob?
Should anyone be upset with Desmond?
This sort of situation seems to baffle people who don't believe ideas can be "property". They start asking other questions like what laws are applicable (copyright? patents?) and whether those laws are just. If they're honest, they start asking if they really understand the concept of property at all.