I spent the day working out a foolproof plan to save him.我花了一整天想出了一个营救他的万全之策。

Abstinence is the only foolproof way to avoid pregnancy.禁欲是防止怀孕的唯一万无一失的办法。

There's still nothing close to a foolproof lie detector.目前还没有不出错的测谎仪。