世界上最長.............

Yes, this is a riddle (謎語).
The answer should be " smiles " .
在前後兩個 s 之間,相隔了一里(mile),所以為之最長的生字。

2007-09-15 19:59:11 補充：
By counting the actual alphabets (字母), the longest English vocab should be:encyclopaedia (百科全書) total 13 alphabets, 又或者是下述專有term:nucleocosmogenesis (核子宇宙進化) total 18 alphabets維基: http://en.wikipedia.org/wiki/George_Gamow

If it is an IQ question...That answer may be:the longest.....(long)
If it isn't....(I don't know= =")