It is often easier to access someone else’s heart than their mind. We can nearly effortlessly pick up on our partner’s mood or sense that a friend dismisses our plans, without them even speaking a word. But how do we know what is going on in their heads? How do we get this special access to the most private of domains—the human mind?

A growing body of research reveals that looking at their eyes may be a neglected and powerful way to do so. The phrases “the eyes are the window to the soul” and “I can see it in your eyes” certainly sound poetic. Many singers, songwriters and writers have capitalized on it. But it turns out that the eyes really might be the windows to the soul. And here’s the great thing about eyes: even if people don’t want you to know how they feel, they can’t change how their eyes behave. So how does this work?

The first thing to look for is changes in pupil size. A famous study published in 1960 suggests that how wide or narrow pupils are reflects how information is processed, and how relevant it is. In their experiment, the two experimental psychologists Hess and Polt of the University of Chicago asked male and female participants to look at semi-nude pictures of both sexes. Female participants’ pupil sizes increased in response to viewing men, and male participants’ pupils increased in response to viewing women.

In subsequent studies, Hess and Polt find that homosexual participants looking at semi-nude pictures of men (but not women) also had larger pupils. This should come to no surprise: after all, pupils can also reflect how aroused we are. But women’s pupils also responded to pictures of mothers holding babies. Hence, changes in pupil size don’t only reflect how aroused we are, but also how relevant and interesting what we see is.

This idea was extended in a 1966 study led by Daniel Kahneman, now a Nobel-prize winning psychologist at Princeton University. He asked participants to remember several three to seven digit numbers, which participants had to report back after two seconds. As participants had to remember longer strings of digits, their pupil size increased, suggesting that pupil size is related to information processing more generally. The first step to know what someone is thinking is to look deeply into their eyes.

In addition to crude information processing, our eyes also convey much more sensitive signals which other people can pick up on. Consider a recent study led by David Lee of the University of Colorado at Boulder who showed participants images of other people’s eyes, and asked them to determine what emotions that person was expressing. It turns out that participants were highly accurate in determining emotions, such as fear and anger, just from looking at images of other people’s eyes.

The eyes can also reveal much more complex phenomena: they can convey whether we are lying or telling the truth. In a 2009 study conducted by Andrea Webb and her colleagues at the University of Utah, one group of participants was asked to steal $20 from a secretary’s purse, whereas a control group did not steal anything. Irrespective of whether participants had stolen the $20, the experimenters asked all participants to deny the theft. Later, by analyzing pupil dilation in response to denying the theft, the researchers were able to tell—better than chance—whether a participant was a thief. Participants who lied about the theft had pupils that were one millimeter larger compared to the pupils of innocent participants.

Our eyes can also serve as a good detector for what people like. To accomplish this, it is necessary to look at pupil size in combination with where someone is looking. Think back to a recent restaurant visit where you had to decide what to eat. These decisions can be simple, for example, if you know straight away what you want. But at other times, such decisions can denote difficult trade-offs, for example when choosing between what you should eat (a salad) and what you want to eat (a burger). Here’s the interesting thing: when the decision is difficult, your eyes are likely to switch back and forth between the different options you are considering, and our last gaze tends to be at the option we end up choosing. And so, by observing where someone is looking, we can infer which options they consider.

One way to study this type of difficult trade-offs is offering monetary bets to participants, such as a 20% chance to win $100 versus a 50% chance to win $40. In a study lead by James Cavanagh of Brown University, participants were asked several questions involving such difficult tradeoffs between payoffs and probabilities. Participants were paid according to their decisions—you can imagine that they thought really hard about which options to choose! The researchers found that the harder the decision—that is, the more difficult the trade-off was between the different choices—the more participants’ pupils were dilated. Our pupils get bigger as choices get harder.

The eyes can also tell us if we experience something unpleasant. In a 1999 study, Chapman and colleagues at the University of Washington administered a painful stimulation to the fingers of 20 participants. Participants rated this pain on a scale from “tolerable” to “intolerable.” The more intolerable the stimulation was rated, the larger participant’s pupils were. Although experiencing pain is very different from looking at semi-nude pictures, it elicits a similar pupil response. Taken together, this suggests that pupil size reflects the strength of feelings, rather than whether those feelings are positive or negative. Therefore, to deduce whether someone is feeling good or bad, we need to consider the context of the situation in addition to their eyes.

Does this mean that we can read everything from the eyes, and that the eyes are the only signal we should attend to? When making a high-stakes decision—such as whether someone was guilty of a crime—we should not rely on pupil dilation alone to make our judgment. It is without a doubt that our “mind-reading capabilities” depend on the context. Generally, you might be better at reading your beloved ones’ eyes compared to a strangers’ because you can tell their regular facial expressions from a surprised one. Converging evidence is the key to help us make better assessments of others’ feelings. But because people can’t change how their pupils react, the eyes are an essential and often under-used source of information that can help us create better bonds with those around us.

It might not be possible to read a person’s exact thoughts from just looking at their eyes. This is great—because from the perspective of the observed, the privacy of thoughts is maintained. But eyes tell us much more than we sometimes assume—and our eyes, unlike our mouths, cannot lie.