There are nine balls. One is slightly lighter than the rest; the difference is small enough that you can’t tell just by picking them up. Using a basic two-sided scale, what’s the minimum number of times you’d need to weigh balls to guarantee you find the light one?
This is one of those questions that, allegedly, gets asked in job interviews. I say “allegedly” because I don’t really believe places really give brain teasers. (Although I haven’t had a job interview since 2003, so maybe they’ve changed, man.)
