The socratic method, which, as the name suggests, he did, is about asking probing questions to expose gaps or contradictions in their knowledge which reveal their ignorance. Both to themselves and oftentimes to other's, as well. He didn't stump them with his own knowledge. He only asked questions until they stumped themselves.

He found that wisdom was found in humility. "The only true wisdom is in knowing you know nothing." That is what I paraphrased as "I dunno". Understanding that you do not and cannot know all things is a core principle of his idea of wisdom and his methodology.