To begin, the instructor explained the modulus function to the class. (The modulus function is a function that divides the second number by the first number and finds the remainder. For example, 8 mod 19 is 3.) After the function was explained, the students practiced using it with the Interactivate applet called "clock arithmetic". After working on those for a while, the class worked out some problems on the board, including a trick question that the students seemed to enjoy.
After break, the instructor started to teach about a type of cipher called the shift cipher, which changes the letters of a sentence to numbers, adds a constant, and changes them back to numbers. After seeing a few examples, the students were given some time to encrypt sentences of their own and try to decipher the other students' codes. After the students had done this at least once, the instructor showed them an applet called the Caesar Cipher. The Caesar Cipher can automatically do shift ciphers, as well as being able to multiply the numbers to make the code more complicated. The multiplying would only work for some numbers, though, and the instructor brought the topic back to the Modulus to explain why even numbers do not work as multipliers.