Here is a perfectly grammatical English sentence: The rat the cat the dog chased killed ate the malt. Almost nobody can read it on the first go. Nothing is misspelled and no rule is broken. The trouble is not grammar. The trouble is waiting.
English lets you tuck one clause inside another. Start with The rat ate the malt. Say which rat: The rat the cat killed ate the malt. That still works. Say which cat: The rat the cat the dog chased killed ate the malt. Each step is legal, and each step is easy to build. But somewhere along the way the sentence stops being something a person can understand by listening or reading once. Linguists including Noam Chomsky and George Miller were pointing this out in the early 1960s: the grammar allows unlimited nesting, while human readers struggle once the nesting goes past a level or two.
The word for this is centre-embedding, a clause sitting in the middle of another clause. The interactive below builds the sentence for you, one level at a time, and shows you what your head has to hold.
Tap a coloured noun or verb to see who did what.
Under each word, a coloured block is a noun you have met but have not yet heard the verb for. The blocks are what you are holding in your head.
Read the nested version aloud with three nested levels and watch the blocks. Every noun at the start opens a loop: the rat needs a verb, the cat needs a verb, the dog needs a verb. No verb arrives. You have to park all three nouns and wait. Then the verbs come in the opposite order, chased, killed, ate, and you must pair the last noun you heard with the first verb, the one before that with the second, and so on, back out. It is like a stack of plates where the last plate on is the first off, and your brain is not good at keeping three unlabelled plates in the air.
Now press Chained. It is the same story told another way: The dog chased the cat that killed the rat that ate the malt. The characters and the verbs are the same, but the stack never grows past one block. Each verb turns up right after its noun, so you can close each loop at once. You can chain forever and nobody notices. That is why children's rhymes such as “This is the house that Jack built” can run on and on.
This is the odd part. Press Drop the middle verb with two or more levels on. You now have three nouns but only two verbs, which is ungrammatical. Yet it can read as fine. The effect was first discussed by Frazier in 1985, who credited the observation to Janet Fodor. Gibson and Thomas gave it a name, the missing-verb effect, in 1999. Their example: The patient the nurse the clinic had hired met Jack. The complete sentence needs another verb after hired: The patient the nurse the clinic had hired admitted met Jack.
In their rating study, sentences missing the first or the last verb were rated as more complex than the complete versions, which fits the idea that readers notice something is wrong. But the sentence missing its middle verb was not rated significantly differently from the complete one. The middle verb is the one your memory has most likely lost track of by then, so its absence is hardly noticed. The sentence feels more fluent without it, and it is wrong.
If you write, the lesson is practical. When a noun and its verb are far apart, the reader has to keep a loop open for the whole gap. Two levels of nesting is a lot, and three is a puzzle. Chain the clauses instead: put each verb right after the thing doing it, and let “that” carry the story forward. One level of nesting is fine, as in the cat the dog chased ran away, and normal speech is full of it. Past that, grammar says yes and the reader says no.
Language does not break when it gets hard to read. It keeps working on paper, where you can look back at the words and count them. What it shows is that the rules of grammar and the limits of the person reading it are two different things.