Some sentences can be all about themselves. One example is:
Only the fool would take trouble to verify that his sentence was composed of ten a’s, three b’s, four c’s, four d’s, forty-six e’s, sixteen f’s, four g’s, thirteen h’s, fifteen i’s, two k’s, nine l’s, four m’s, twenty-five n’s, twenty-four o’s, five p’s, sixteen r’s, forty-one s’s, thirty-seven t’s, ten u’s, eight v’s, eight w’s, four x’s, eleven y’s, twenty-seven commas, twenty-three apostrophes, seven hyphens, and, last but not least, a single!
It could be fun to try to create the structure and coding to generate something like this! But here is an easier [but still plenty hard, IMO] one:
In this sentence, the number of occurrences of 0 is _, of 1 is _, of 2 is _, of 3 is _, of 4 is _, of 5 is _, of 6 is _, of 7 is _, of 8 is _, and of 9 is _.
Fill in the blanks to create a true statement. Just to keep you from going down some rabbit holes, I will state that the numbers are in base 10, and that the two solutions of which I am aware both have single digits in each blank.