Funny, but it was just at the excerpted part where I started to lose it. I was with them until they started talking about the falling rocks.

I found the 4 letter word limit actually hinders understanding. It was cute for awhile, but not really practical. If they could've used a word like "light" instead of "ray" it would've been smoother. It seemed at times awfully contrived to stay under the letter limit.

Ah, but the "most common" words are not necessarily enough
to make much sense. The Beginner Books method, I believe,
was two hundred common words plus twenty others selected
by the author to bridge the gaps.

So then lets build it from the 300 most common words - theoretically these would also translate into other languages better as well.

"the of and a to in is you that it he for was on are as with his they at be this from I have or by one had not but what all were when we there can an your which their said if do will each about how up out them then she many some so these would other into has more her two like him see time could no make than first been its who now people my made over did down only way find use may water long little very after words called just where most know

get through back much before go good new write our used me man too any day same right look think also around another came come work three word must because does part even place well such here take why things help put years different away again off went old number great tell men say small every found still between name should Mr home big give air line set own under read last never us left end along while might next sound below saw something thought both few those always looked show large often together asked house don't world going want

school important until one form food keep children feet land side without boy once animals life enough took sometimes four head above kind began almost live page got earth need far hand high year mother light parts country father let night following two picture being study second eyes soon times story boys since white days ever paper hard near sentence better best across during today others however sure means knew it's try told young miles sun ways thing whole hear example heard several change answer room sea against top turned three learn point city play toward five using himself usually "

Four or fewer letters per word is not a good metric for
being understandable. Limiting the vocabulary to a small
collection of well known words is what is needed. Look at
the Dr Seuss classics. Green Eggs and Ham reportedly
has 50 words (I have not counted*, but it is clearly less
than 100), many of those are longer than four letters, though.

* The Cat in the Hat reportedly uses 220 words, but
I counted 235. So until I count GE&H I won't trust the count.

Okay, my bad. I just followed the link and the sections that weren't excerpted do indeed go on to add concepts from the general theory.

Slight nitpick: At least for the excerpt presented here, the relevant theory is the _Special_ Theory of relativity. The _General_ theory adds the concept of spacetime curvature to explain gravity.