Reading about any mathematical topic on Wikipedia. For example, https://en.wikipedia.org/wiki/Limit_superior_and_limit_inferior https://en.wikipedia.org/wiki/

Reading about any mathematical topic on Wikipedia. For example, https://en.wikipedia.org/wiki/Limit_superior_and_limit_inferior https://en.wikipedia.org/wiki/Limit_superior_and_limit_infer... --> https://en.wikipedia.org/wiki/Partially_ordered_set https://en.wikipedia.org/wiki/Partially_ordered_set --> https://en.wikipedia.org/wiki/Binary_relation https://en.wikipedia.org/wiki/Binary_relation --> https://en.wikipedia.org/wiki/Real_number https://en.wikipedia.org/wiki/Real_number --> https://en.wikipedia.org/wiki/Cauchy_sequence https://en.wikipedia.org/wiki/Cauchy_sequence ... Even if I think I know what's being discussed in the article, there's always some interesting extra detail or alternative way of explaining things that's worth reading.