1. To inform someone of the facts or several pieces of information (about someone or something). I know you're from the countryside and not used to city folk, so let me tell you a thing or two about New Yorkers. If you want to learn about the history of cinema, then you should ask Jeff—he'd be more than happy to tell you a thing or two.

2. To correct or confront someone about his, her, or their mistaken belief or incorrect point of view (about someone or something). The professor is teaching us some really antiquated material about quantum physics. I think I'll go tell him a thing or two about it after class. That jerk has been making snide remarks about the women in our group all night. I'm going to go over there and tell him a thing or two!