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Code-Doku Joke

What do you call a song sung in an automobile?

Place each of the letters of CAST IRON exactly once in each row, column and 3×3 box. In addition to the letters, each row, column and 3×3 box will also contain one empty square. When you have completed the puzzle, the shaded squares will form the punch line of the joke read left to right, then top to bottom.

Difficulty: Easy

Here is a print-friendly version of this puzzle.

Other difficulties:
All of these can be solved using the sudoku placement rules alone, and they all have the same hidden message. Choose a puzzle difficulty that fits your interest, or solve the easy puzzle then try a harder one after you know the message.

Please send responses to my email address is mathrec at this domain