Next time you are speed dating, don’t count a woman’s hair flips or smiles to see if she’s interested—listen to her words. By looking at your conversation, you can predict your likely success, according to psychologist James Pennebaker. Knowing whether a speed dating interaction will lead to a real date, however, doesn’t depend upon the topic of the conversation. Pennebaker doesn’t look at whether you both enjoy football, or whether you ask about her hobbies. The real truth lies in what no one ever notices—the filler words. Sentences are made up of content words—football, dance, music—and function words—the, I, an, there, though, that, this. Function words tie sentences together, but they are essentially invisible. "These are the words that we don't pay attention to, and they're the ones that are so interesting,” Pennebaker told NPR. By using a computer program, he analyzed how people use both content and function words. In a speed dating context, Pennebaker was able to determine future dating success. "We can predict by analyzing their language, who will go on a date—who will match—at rates better than the people themselves," he said. This is not simply because people who are similar—and talk the same way—are attracted to each other. When we talk to people, our language shifts. These changes are not conscious, but happen automatically. It turns out that when a speed dating couple’s language style matched—in terms of pronouns, articles, prepositions, etc.—they were more likely to go on a date. Pennebaker's method was so effective, he was also able to determine whether they would still be dating three months down the line.