Palindromes.

Palindromes grow more impressive as they grow longer, because every added word must mirror itself against a partner across the centre. The classics are famous for a reason: Madam, I'm Adam; Never odd or even; Was it a car or a cat I saw?; and the most quoted of all, Leigh Mercer's A man, a plan, a canal — Panama! Punctuation, spaces, and capital letters are forgiven; the letters themselves must match exactly in reverse.

Look closely at how the long ones are built and you will find levidromes doing the heavy lifting: a sentence palindrome very often pairs a word with its own reversal across the middle, the way live and evil mirror each other in John Taylor's 1614 line 'Lewd did I live, evil I did dwell' — among the first palindromic sentences in English. The full story, from the Sator Square carved at Pompeii to the Panama sentence, is in the Journal: The Long History of Palindromes.

The test takes two seconds. Reverse the word. If you get the same word back — LEVEL, KAYAK, NOON — it is a palindrome. If you land on a different word with a different meaning — LEVEL’s neighbours LIVE and EVIL, STOP and POTS — you have a levidrome.

The two families are close cousins, and the shared ending is no accident: levidrome was coined from Levi Budd’s first name plus the -drome of palindrome. Read the whole story in What Is a Levidrome?, or browse the 48 levidrome pairs.