You are constantly learning new things, remembering previously-learned things, and forgetting old things. Many aspects of learning and memory follow patterns that are quite consistent and predictable, yet we often don’t notice when these effects are happening.

The most basic form of learning is called classical conditioning or Pavlovian conditioning. You may have heard of the famous example of Pavlov’s dog, where where Pavlov would ring a bell every time he fed his dogs, and the dogs started salivating when they heard the bell even when there was no food. In classical conditioning, you learn to associate some stimulus (such as a bell) with a result (such as food). In another famous experiment, a baby known as Little Albert would play with a white rat, and then the experimenters started banging a loud metal bar every time he touched the rat, and after a while he started crying whenever he saw the rat. Similar effects are influencing your own associations in more subtle ways all the time - for example, advertisements teach you to associate pictures of happy attractive people with particular products.

A similar form of learning is called operant conditioning, in which you learn that a certain action causes a certain consequence. This is one of the ways parents teach their children to behave in certain ways - you get rewarded for doing things your parents approve of, and you get punished for doing things they disapprove of, so you learn that there will be better consequences when you do what your parents want. There’s one experiment where a classroom of students decides that whenever the teacher moves toward one side of the room they’ll smile and act interested, and whenever the teacher moves toward the other side they’ll act bored; by the end of class, the teacher will be standing as far as possible toward the side that makes the students smile, probably without having even noticed why.

Some things are learned more easily than others though - you don’t automatically associate things just because they occur together. There was an experiment on rats where they gave the rats flavored water and also flashed bright lights and made loud sounds, and then either gave the rats electrical shocks or a stomachache; the rats learned to associate the flavored water with getting a stomachache and to associate the light/sound with getting shocks, but they didn’t learn to associate the flavored water with getting shocks or to associate the light/sound with getting a stomachache.

How well we remember things is not always the same as how well we think we remember them. Sometimes when you have a particularly surprising or emotional experience, you feel like you remember it very vividly, but you probably don’t actually remember it better than you remember normal things; this is called a flashbulb memory. Sometimes you can even feel like you remember something in detail when it didn’t actually happen! Experimenters tried asking people if they had been to Disneyland and whether they had met Bugs Bunny, and many of the people claimed to remember that they had (in real life they couldn’t have, because Bugs Bunny is not a Disney character); the effect was especially strong when there was a cardboard cutout of Bugs Bunny in the room during the experiment. Here is a related demo that you can try.

The “amount of stuff” you can remember for a short time depends on how your mind is representing it. It’s a lot easier to remember “123456789” than “938571264”, even though they’re both just the same nine digits in different orders, because the first one can be represented as a single thing you already know. Chess masters are better than the average person at memorizing chess board configurations, but they’re worse than the average person at remembering configurations that can’t actually happen in a game of chess.

In general, the more your brain processes something, the better you remember it. For example, in one experiment, people were given a list of words, and one group of people had to say whether each word was written in all capital letters, a second group had to say whether each word rhymed with a certain other word, and a third group had to say whether each word fit into a certain sentence; the third group remembered the words better than the second group, who remembered them better than the first group. This can be counterintuitive because when you go through something easily, it can make you feel like you’ll remember it really well, but actually it can indicate that you didn’t have to think about it very much. You may have noticed this effect when studying for a test: if you study by just reading through your notes or example problems, you might feel like everything makes sense but then find that during the test you can’t remember things, whereas if you study by doing practice problems, your brain has to process and use the information, and you might feel like you don’t understand things but you’ll probably do better on the test. Similarly, learning something in several sessions spaced out over time actually works better than repeating it many times in a row; this is called the spacing effect.

Different things can be easier to remember in different contexts. Often, it’s easier to remember things in the same context where you learned them - for example, you may have had the experience of sitting at your desk and then getting up to do something, only to realize you’ve forgotten what you were going to go do, and then remembering it once you get back to your desk.

There are different ways in which you can “remember” something - if someone asks you whether the Mona Lisa painting is facing to the left or the right, you might not be able to say, but if you look at both versions, you can probably tell which one looks correct. This is the difference between “explicit” and “implicit” memory, and we know that these are different things because it’s possible for someone to have amnesia and not be able to remember new things explicitly but still remember them implicitly. One doctor did an experiment where he introduced himself to a man with amnesia and when they shook hands he was holding a tack, which poked the man’s hand; when the doctor left the room and came back, the man with amnesia didn’t think he’d ever met him before, but when the doctor reached out to shake the man’s hand, the man didn’t want to - he implicitly remembered being poked by the tack.