• The cat caught the spider that caught the fly the old lady swallowed.


• The fly swallowed by the old lady was caught by the spider caught by the cat.

• The fly the spider the cat caught caught was swallowed by the old lady.

The first two sentences are hard to understand, but make some kind of sense.

The last sentence is merely rearranged but makes no natural sense at all. (This is all assuming it makes some sort of sense for an old lady to be swallowing cats in the first place, which is patently absurd, but it turns out she swallowed a goat too, not to mention a horse, so we’ll let the cat pass without additional comment.)