Geniuses might be distinguished by their ability to grasp incredible complexity, but that doesn't mean if you somehow managed to corner one the greatest minds in history for a chat you'd be perplexed by what they had to say. According to Nobel Laureate Richard Feynman, the true hallmark of genius is the ability to explain things simply.

If that's true, it's yet another sign of Albert Einstein's incredible gifts (as if we needed further proof).

A glimpse of Einstein as doting dad

In 1915, Einstein was living in Berlin and working on his theory of general relativity while his estranged wife tended his two sons in Vienna. In an age before email and Skype, that meant a regular exchange of letters between the great physicist and his family, one of which was recently dug up by Maria Popova of the always intriguing Brain Pickings blog.

The short note to 11-year-old Hans Albert Einstein not only shows Einstein in a less familiar light as a caring father, but also illustrates Feynman's point -- geniuses don't talk in riddles but in language that's exceptionally clear.

In this case, Einstein uses this forceful simplicity to offer young Hans Albert -- and all of us readers listening in a hundred years later -- some exceptionally good advice on how to learn more quickly:

I am very pleased that you find joy with the piano. This and carpentry are in my opinion for your age the best pursuits, better even than school. Because those are things which fit a young person such as you very well. Mainly play the things on the piano which please you, even if the teacher does not assign those. That is the way to learn the most, that when you are doing something with such enjoyment that you don't notice that the time passes. I am sometimes so wrapped up in my work that I forget about the noon meal. . . .

Unsurprisingly, Einstein's intuition that passion, flow, and even laughter are far better study aids than the usual dreary suggestions about highlighters and flash cards have subsequently been proved by research (though there's no word from child psychologists on the exceptional benefits of carpentry, at least as far as I've heard).