1. A sentence must contain forty letters to be true.
2. This sentence does have a total of forty letters.
3. Therefore the second sentence is a true sentence.

This logic seems to hold up. Unfortunately, sentences 1 and 2 have 40 letters each — and sentence 3 has 41.

(David Morice, “Kickshaws,” Word Ways 22:1 [February 1989], 44-52.)