Eyes are the windows to the soul, they say. Well, they weren’t kidding! New data suggests that measuring the size of the pupil—a science called “pupillometry”—can reveal how captivated someone is by what they’re seeing. Just as our pupils expand in dim light, they widen when we’re looking at something that captures our attention, says a new report in Perspective on Psychological Science.

Scientists aren’t sure why this happens, but the potential applications are endless. In one study, researchers measured pupil changes as they showed new and repeat pictures to amnesia sufferers; pupils widened at new pictures, even when subjects couldn’t tell the difference. Similarly, measuring a baby’s pupils could reveal concepts he recognizes before he can even articulate it. Bottom line: on your next blind date or business meeting, use this little pupil trick to glean what they’re really thinking.

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