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You might think that babies don't do much besides eat and poop. ...and maybe that's kind of true. But before they can talk or walk, infants start to do all sorts of cognitive feats that seem awfully smart for a baby.

Inside their little brains, there's a lot more going on than you might expect. In the first three months of a child's life, their brain nearly doubles in size, and it continues to grow at break-neck speed until they're 3. But what all that new brain matter is doing is a little harder to determine.

You can't ask babies to solve puzzles or tell you what they're thinking, so researchers have had to get creative to study infant cognition. They can examine what infants do, for example, like what behaviors they choose to imitate or which people or objects they interact with. And even before they can move around or make gestures, scientists can use the violation-of-expectation paradigm to peer into babies' minds.

Put simply, infants look differently at things that are surprising or unexpected so by measuring how long they stare at different scenarios or how their pupils change while they stare, scientists can generally tell what the baby thought would happen. Researchers can also use high-tech brain imaging techniques like functional magnetic resonance imaging or fMRI, which looks at blood flow to determine which brain areas are most active, or electroencephalography or EEG, which measures the electrical patterns generated by brain cells firing. And by using some or all of these methods, psychologists have discovered infants have a lot going on in those tiny little noggins.

Infants as young as 9 months old seem to do simple math, for instance. In a 2004 violation-of-expectation study, babies watched a movie where 5 objects fell behind a wall, followed shortly by 5 more. Then the wall moved off screen, revealing either 5 objects, the wrong answer, or 10 objects, the right answer.

The babies stared longer at the screen when only 5 were revealed, suggesting that they knew that 5 plus 5 should equal more than 5. Similar studies have shown that infants as young as 2 months seem to have a rudimentary grasp of physics. At that age, they know where a moving object should end up, even if it goes behind a barrier.

By 4 to 5 months, they expect unsupported objects to fall rather than hover, suggesting they are aware of the phenomenon of gravity. And at 7 months, they'll look longer at a ball that rolls up a hill than a ball that rolls down it. Babies even seem to recognize differences between the states of matter.

In experiments, they seemed to expect liquids to flow through a grate and expect solids to stay on top. All this makes sense because to survive as adults, humans need to learn about the properties of the physical world around them. And babies sure get lots of time to study how things behave, since they can't, you know, do much else.

Young humans also need to learn the unspoken rules of human society and develop an understanding of things like emotion, language, and cooperation. And that learning starts early, too. Most infants seem to pick up on emotions really early on, for example.

When scientists scanned the brains of 3 to 7 month-olds while they listened to emotional voices, they found that sad voices produced more activation in brain regions which are important for emotion processing, like the insula. And well before they say their first words, babies seem to be laying down the foundations for understanding language. Even at 3 or 4 months old, studies suggest babies use novel words that they hear as a way to categorize objects, an effect that does not occur when they hear a simple tone instead.

And they seem to recognize and value certain social traits like collaboration. In a 2007 study, 6 to 10 month-olds chose to reach towards a character that helped another character climb up a hill rather than one that pushed others down, indicating that they understood and preferred cooperative, or prosocial, behavior. There's even some evidence babies can get inside other people's heads.

Even though infants aren't generally considered to have theory of mind, the perception that other people have their own unique set of thoughts, feelings, and beliefs, they do seem to make basic inferences about why other people do what they do. For example, in one study, researchers had 14 month-old infants watch as an adult turned on a light with her head. The woman either had her hands free, or pretended to be cold and wrapped herself in a blanket which prevented her from using her hands as normal.

When her hands were occupied, the babies were less likely to imitate her, and instead turned the light on with their hands. But when her hands were free, they followed her lead, turning the light on with their heads, too. It was as if they understood that, if her hands were occupied, she was turning the light on in a strange way because she was unable to use her hands.

But if her hands were free, then she used her head because, for some reason, that was the best way to do it. So even though it seems like they're not doing much, long before babies can talk or even walk, they're learning a lot about the world around them every day. And that's probably why it seems like they grow up all too fast.

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