In 1984, Barry Marshall was a young Australian doctor who had a theory that most stomach ulcers were caused by bacteria — specifically, a bacteria called H. pylori. The medical establishment disagreed completely. Doctors had believed for decades that the stomach was too acidic for bacteria to survive. Ulcers, they said, were caused by stress and bad diet. Marshall could not get anyone to listen. He submitted papers. He was rejected. He tried clinical trials. Nobody would fund them. So he drank a petri dish full of H. pylori. He developed severe gastritis within days. He then treated himself with antibiotics and recovered. The experiment worked on the most credible possible test subject: himself. He eventually won the Nobel Prize in Medicine. This story illustrates the fourth principle of Made to Stick: Credible. Ideas need to be believed before they can be acted on. But most of us are not Nobel laureates, senators, or CEOs. We do not have credentials. How do we make our ideas believable without authority? The Heaths identify several strategies. The first is anti-authority. Sometimes a non-expert who has lived through something is more credible than any expert who has studied it. A former smoker talking about lung disease is more persuasive than a statistics table about mortality rates. A young person who grew up without clean water is more convincing on water access than an academic paper. The second is vivid, specific detail. Research shows that specific details — the kind that are oddly precise — signal that someone has been there and seen it. A claim with concrete particulars feels more credible than a vague general statement. The third is what the Heaths call testable credentials: Ronald Reagan's most powerful line in a 1980 debate was not a statistic. It was a question he invited Americans to answer for themselves: before you vote, ask yourself if you are better off today than you were four years ago. He gave people a way to test the claim with their own experience. Adaeze is pitching her community health project to local funders. She does not have a degree or published research. But she can say: I spent six months going door to door in this neighbourhood. In three streets alone, I found fourteen women who had never had a cervical screening. One of them was Mama Chioma at number 7, who thought the service did not exist for people like her. That specificity — the name, the number, the street — is credibility.