The One-Time Pad (OTP) is an unbreakable encryption method — as long as the key is truly random, used only once, and kept secret.
But when the same key is reused to encrypt two messages, the security collapses. By XORing the two ciphertexts together, the key cancels out, leaving only the XOR of the two plaintexts. From there, the Crib Drag attack slides guessed words across the result, revealing plaintext without ever knowing the key. Read more here.
Want to try it yourself? Use the OTP Encoder to create your own pair of ciphertexts, then come back here and crack them.
Result: