What do you call a word that reads the same backwards?

A palindrome, from Greek meaning running back again. Single words are easy - level, rotor, kayak, racecar - and sentences are where it becomes a sport: A man, a plan, a canal: Panama, or the one usually put in Napoleon's mouth, Able was I ere I saw Elba.

They are much older than the wordplay tradition suggests. The Sator Square, a Latin word square that reads the same in four directions, has been found scratched into walls at Pompeii, so it predates AD 79 and was still being copied into medieval manuscripts as a charm.

Some languages take to it far better than others. Finnish and Hungarian, with their long strings of vowels and agglutinated endings, produce enormous natural palindromes. And there is a genuine scientific use of the word: palindromic sequences in DNA, where a strand reads the same as its complement in reverse, are exactly what restriction enzymes recognise and cut - which is what made genetic engineering possible in the first place.