Great question, Florrie. Both “join” and “join in” can refer to activities, but there is a difference in how we use them. Join usually means becoming part of a group, event, or organized activity. For example: “I joined a photography club.” “Would you like to join us for dinner?” “We joined a guided tour of the city.” “The students joined a school tour.” In the last example, “join a school tour” is correct and natural because the students are becoming part of an organized group or event. Join in emphasizes participating in an activity that is taking place, often alongside other people. For example: “Everyone was singing, so I decided to join in.” “The children joined in the game.” “She joined in the conversation.” “We joined in the celebrations.” The important distinction is that “join” focuses on becoming part of something, while “join in” focuses on actively participating in what people are doing. So, although a school tour is an activity, we would normally say “join a school tour” because we are talking about becoming part of the tour group. You could also say “take part in a school tour” or “go on a school tour.” The rule you learned is a useful starting point, but it is not absolute. We can use “join” for organized activities as well as groups. I hope that helps.