There are several common English 6-letter words that use only 3 different letters. For instance, BANANA, NEEDED, MURMUR...
Can you think of a common English 7-letter word that uses only 3 different letters?
Bonus: How about an 8-letter word? (I know of at least 2 common ones.)
Ready for the answer?
Did you get it?
I know of a few common(-ish) 7-letter words: REFEREE, ALFALFA, GINNING, GIGGING, PREPPER (one who prepares, a survivalist).
And the 8-letter words: ASSESSES and REFERRER (something that refers you to something else).
For the HTML geeks among us, REFERRER is permanently misspelled as REFERER (note the missing R) in the HTTP specification. That's so pervasive that might even become an acceptable spelling someday.