Time flies; you cannot; they pass at such irregular intervals.

The line is presented as a punctuation/interpretation puzzle built around syntactic ambiguity. It first appeared in a reader-interaction column in a London periodical in 1903, where the columnist using the pseudonym “Mr. X” challenged readers to punctuate and interpret the unpunctuated sentence. A later issue supplied the intended reading, explaining that “time” is used in the sense of timing a race and “flies” refers to insects, not the idiom about time passing quickly.

The joke depends on re-parsing “time flies” as an instruction to measure the speed of insects. With the right punctuation, the sentence becomes a complaint that you can’t successfully time them because their movement is erratic.

Time flies; you cannot; they pass at such irregular intervals.

Time flies; you cannot; they go too quickly.

Time flies you cannot; they pass too quickly.