How do you measure nine minutes with a seven-minute and a four-minute hourglass?
Flip both of 'em over at the same time. When the four minute runs out, flip it over (four minutes have elapsed so far). When the seven minute one runs out, flip it back over. The four minute timer should flip over by now (eight minutes). Flip the seven minute timer over. It only has been running for 1 minute. Boom. 9 minutes.
Nice job.
Google's gonna kill this forum though.
http://www.google.com/search?sourceid=navclient&ie=UTF-8&rls=GGLG,GGLG:2005-23,GGLG:en&q=How+do+you+measure+nine+minutes+with+a+seven%2Dminute+and+a+four%2Dminute+hourglass%3F
My math teacher in 7th grade told us riddles. :P
But yeah, Google is a bitch.
For the google problem, just reword the original riddle enough that it's difficult to search for.
Hopefully, people won't use Google here; these are for fun, not really for a contest :)