A farmer has seventeen sheep. All but nine died. How many sheep does he have left?

“Seventeen”

The riddle states that all but nine died, meaning nine sheep are still alive. If all seventeen were alive, then the statement 'all but nine died' would not hold true.

“Eight”

If eight sheep were left, that would imply that nine sheep died, which contradicts the riddle's statement that all but nine died.

“Six”

This answer suggests that more sheep died than are actually left. The riddle clearly states that nine sheep survived, making six an incorrect guess.