"I am your identity written on every surface you touch. No two of me are alike in the entire world. I have been used to catch criminals and identify the dead. What am I?"

Answer: A Fingerprint. Fingerprints — the patterns of ridges on fingertips — are unique to each individual, including identical twins. They are left on almost every surface touched. Fingerprint analysis has been used in forensic identification since the late 19th century.