Our pupils, the black holes which let light into the eyes, don't just help us see, they also signal what's going on in our minds.
Here are 10 pieces of psychological research which show how changes in pupil size reveal many aspects of thought.
1. I'm thinking hard
Look into my eyes and ask me to name the cigar-smoking founder of psychoanalysis and you won't see much change in my pupil size. The name Sigmund Freud comes easily to my lips.
But ask me to explain the laws of cricket and watch my pupils expand.
That's because research has shown that the harder your brain works, the more your pupils dilate. When Hess and Polt gave participants more and more difficult tasks to complete, their pupils got bigger and bigger.
My brain is overloaded
Keep watching my eyes closely and you'll spot the point when explaining the laws of cricket gets too much.
Poock reported that when participants' minds were loaded to 125% of their capacity, their pupils constricted.
It'll be trying to explain a googly that will do it. (Don't ask).
3. I'm brain damaged
The reason doctors and paramedics flash a light in patients' eyes is to check their brains are working normally (and because it's such an easy test to do). They use the acronym PERRL: the Pupils should be Equal, Round and Reactive to Light.
为什么医生和护理人员们会用一个手电筒照眼睛？为了检查脑功能是否正常。它操作起来非常简便易行。他们用PERRL的缩写来表示：即，瞳孔(Pupil)应该双侧等大(Equal)，等圆(Round)，对光反射存在(Reactive to Light)。
If my brain is broken, say, because I've had a bump on the noggin, you won't see PERRL. There may well be other extremely subtle clues, like the blood pouring from my head.
4. You're holding my interest
The size of my pupils can also signal whether I'm interested in what you're saying.
White and Maltzman had participants listening to excerpts from three books: one was erotic, another involved mutilation while a third was neutral.
Their pupils widened at first for all three. But they only remained wide for the passages that were erotic or involved mutilation.
I'm likely to be interested in anything new, so my pupils will dilate a bit at first, but they'll only stay dilated if I continue to be interested.
5. You're turning me on
If things take a sexual turn then our eyes are also involved. Both men and women's pupils expand when they are sexually aroused.
However, not everyone agrees big pupils are a signal of sexual arousal. It tends to get tested by showing nude pictures to people and some argue that we're just really interested in the nude form.