The easiest lesson learned is one that is is self-taught.
How do you learn something? I mean really well and truly.
Well for me there are two ways.
They both have to do with experience.
That's right, experience. You have to experience something to know.
Fine and dandy if you're told the pan's hot. Well yes, but maybe you don't believe it. So...you go check. Youch! It is hot. That's what I mean by experience. You have experienced the heat of the pan for yourself, therefore you know it's hot.
And will you ever forget that pans on the stove are usually hot? Not likely.
Another example...you like to ride your bicycle. After a few days, you realise that the footpaths near your house are extremely bumpy. And you get sore. Then you have a stroke of genius! Stand up on the bumps so you don't absorb the impact. Yay!
You rush home to tell your folks. You excitedly pour out the story. Then your dad says 'I told you that ages ago.' And so he did. Why didn't you remember? Because it wasn't important. Why wasn't it important? Because it wasn't relevant. You will always remember to do that because you taught it to yourself, no matter how many times you were told before.
The other way to learn it; embarrassment. Say your teacher calls on you in class. You don't know the answer. The teacher then spends half an hour lecturing the class on studying. Are you likely to forget that answer? Well, maybe. But if the teacher then calls on you a second time, days later and you still don't know the answer...You've been embarrassed twice (it only works for some people! Some might not get embarrassed and end up having a laugh instead, it depends on the person. Then again, someone might do both). Do you really want the experience of being embarrassed again? For me, it takes being embarrassed three times, then I know it. Really well. Really really well.
I suppose a third way could be added; endless recitation. But that's no fun, and could be forgotten. Learning things that interest you are good as well, because if it's interesting, you'll remember it, right?