Designing engaging exercises when students do not yet possess a lot of knowledge can be difficult. We show how we draw on students’ prior knowledge, along with basic introductory concepts, to design an elemental (but fun) port scan exercise in an introductory security testing module. While “capture the flag” is a security industry standard for exercises, it can require a lot of in-depth knowledge to properly implement and complete. Using basic computer science concepts such as ports and ASCII values, we design a simplified capture the flag exercise where students can make use of deductive reasoning to complete the game. Overall, the exercise was received favourably by the students who found it challenging but enriching.