The students wrote each letter of the alphabet vertically on a piece of paper. They then created expressions containing at least one set of parentheses and two different operations (I did this while teaching 4th grade, so that was the standard...in 5th I would probably add negative numbers or decimals into the mix). It looked a bit like this:
A (5 x 2) - 4
B 2(4 + 3)
C 15 - (2 x 3)
The students then solved each expression to make sure the answer was unique (no two letters could have the same end product). This part is VERY challenging for the students. It actually took them two days. The process of thinking of problems, and then making sure they had unique values, really got their 4th grade brains moving! Once all of the expressions were created, they did not reveal the expression answers on the final draft. Those were turned in to me. Their cipher key just had the letters and expressions.
The next step was to write a sentence that had to do with communication (since that was our Language Arts theme...I like to connect things). The sentences needed to be at least 7 words long, but could convey any message within the theme. They then drew the blank lines for each letter in the word, and put the number of the final expression answer underneath. So it looked like this:
_____ _____ ______ ______ ______
14 6 12 75 32
Finally, they created a finalized draft of the secret code, with the cipher key above it, and viola, there were 30 codes to be solved!
Once they were all created (and checked for validity by me) I made the board interactive by placing index cards right next to the codes. During Math Workshop, students could go to the board, grab an index card, and begin to solve the expressions to decipher the code. (they put their answers into the box that is stapled onto the board under the detective) A LOT of math that the kids really never got bored of!