Sometimes you just don't have enough time to read an entire proof, a brief scan is all you can afford. Probabilistically checkable proofs (PCPs), discovered 25 years ago, guarantee that even a brief scan will find an error if there is one. A PCP proof is created by taking a regular proof and splitting it cleverly into fragments. The key is a theorem asserting that locally consistent fragments must be coming from a globally correct proof. We will describe this surprising local-to-global phenomenon and show a variety of implications from computational optimization all the way to secure cloud computing.