Are tongues a sign for believers or unbelievers?

1 Corinthians 14:22-25 confuses me. Verse 22 states: > **Tongues**, then, are a sign, not for believers but for **unbelievers**; prophecy, however, is not for unbelievers but for believers. So, tongues is a sign for unbelievers. But then, verses 22-25 go on: > So if the whole church comes together and everyone speaks in **tongues**, and inquirers or **unbelievers** come in, *will they not say that you are out of your mind?* But if an unbeliever or an inquirer comes in while everyone is prophesying, they are convicted of sin and are brought under judgment by all, as the secrets of their hearts are laid bare. So they will fall down and worship God, exclaiming, “God is really among you!” These two parts seem in contradiction. And this also applies (in reverse) to Paul's comments on prophecy. Can anyone explain this?