In trials witnesses often gain by slanting their testimony. The law tries to elicit the truth from witnesses by cross-examination under threat of criminal prosecution for perjury. As a truth-revealing mechanism, perjury law is crude and ineffective. We develop the mathematical form of a perfect truth-revealing mechanism, which exactly offsets the gain from slanted testimony by the risk of a possible sanction. Implementing an effective truth-revealing mechanism requires a witness to certify accuracy by posting bond. If events subsequently prove that the testimony was inaccurate, the witness forfeits the bond. By providing superior incentives for telling the truth, truth-bonding could combat some distortions by factual witnesses and interested experts, including “junk science”.