Something that is foolproof is designed so simply and carefully that it cannot fail, even if someone makes a mistake or doesn't understand exactly what they're doing. A foolproof plan has no weak spots. Foolproof instructions are so clear that anyone can follow them successfully.

When scientists send a spacecraft to Mars, they try to make the landing systems foolproof because there's no way to fix problems once the spacecraft is millions of miles away. A foolproof recipe might be one that comes out delicious even if you're not an experienced cook. A foolproof lock is one that can't be picked or broken into.

The word suggests careful design that accounts for human error. Engineers build foolproof safety systems into airplanes, knowing that even skilled pilots sometimes make mistakes under pressure. Teachers create foolproof procedures for fire drills so that even kindergarteners know exactly what to do.

Nothing is truly completely foolproof, but calling something foolproof means it's been thought through so carefully that failure is extremely unlikely. When you're working on a project, trying to make your approach foolproof means thinking ahead about what could go wrong and preventing those problems before they happen.