The riddle
How Do You Make Seven Even?
Spell it out
Write the number as a word: s-e-v-e-n. Take away the first letter and e-v-e-n is left, the word for a number that divides by two with nothing left over. Wikipedia's article on parity defines an even integer as one that is divisible by 2 and notes that 7, which equals 2 times 3 plus 1, is odd. The riddle never touches the arithmetic. It changes the word. The arithmetic route also works, since adding or subtracting 1 gives 8 or 6, both even, but the riddle works on the spelling instead.
Why the digit does not work
Searchers often type the question with the digit 7, as in "how do you make 7 even". The digit has no letter to remove, so the answer needs the spelled word. Some versions of the question forbid adding, subtracting, multiplying and dividing, and the same answer still holds, since it does none of those things.
Only seven can do it
Try the same move on the other odd number words. One becomes ne, three becomes hree, five becomes ive, and nine becomes ine. Only seven leaves behind a real word that fits. That is why this riddle is always about seven and never about any other odd number.
Older printings
A rhyme in Riddles and Jokes (London, 1863) asks about an odd number that loses its first letter and ends, "It then will even be", with the answer "Seven." J. Gilchrist Lawson's 1924 collection asks which odd number becomes even and answers "Seven" too. Another number riddle built on letters is how do you spell cow in thirteen letters.