Handling "contraband" tattoos in closed-book exams I'm wondering how a proctor of a closed-book exam should approach the case where a student has a tattoo that would double as a cheat sheet. Maybe they think the math looks cool, or they got a nice calligraphy of a textbook passage, or they are just that desperate to cheat.

Now, on one hand, that tattoo is pretty much a permanent fixture. On the other hand, that is a cheat sheet written on your forearm. In fact, it might be a particularly brazen attempt at cheating. And on the gripping hand, the fact that a resource is available for easy reference whenever needed doesn't carry much weight in the context of a closed-book exam.

At least in my university, the general rule for exams is "No written material unless expressly permitted", and most of the exams that permit bringing a "cheat sheet" have had a restriction on the sheet size (e.g. one-sided hand-written A4 paper). Therefore, a situation may well arise that a student shows up for an exam with the maximum-sized cheat sheet, and some related math (purportedly) tattooed on their forearm. If I find myself proctoring an exam and this happens, how should I approach the situation?

This is an entirely hypothetical question so far, so I'm unable to provide details of a real-life example.
"Cover it up" seems the only realistic solution if it appears to be a tattoo.

However, since we're discussing this in the hypothetical, writing a theorem on your arm may serve as a nice reminder of a basis from which to work on problems, but it can't really answer something useful for you. Just as younger students tend to look at their fingers when counting, they still have to put in the work of translating an abstract numerical problem to their fingers, then "read" the information from their hand, and finally write their answer down. We don't restrict their fingers as counting tools. Advanced students make more elaborate associations between concepts they're learning and something concrete -- this is usually encouraged by instructors when we explain things in plain language or metaphors. So, your question makes me think about what questions I'm asking my students. If it's just "Write down this definition from memory", then maybe I'm not going deep enough in the topic. Suppose instead that I said something like "Here's a definition of a key term you can look up anywhere, or tattoo it on your arm if you really like it! Now, which of the following things matches this description?"