A long message, enciphered with a keyword you don't know. Guess the key length, look at every N-th letter, and read the keyword out of the peaks.
Paste a ciphertext whose key you don't know, or paste a plaintext and a key to make a fresh challenge. Letters only are used; everything else is dropped.
When N is right, or a multiple of the right length, every stream is a Caesar cipher and its IC jumps toward English.