I start by writing four words on the board:
And then ask the students the key phrase "Which is different?"
Eventually a student will say "Kendo." and I'll circle it and then ask "Why?" Often the student will explain in Japanese, so I'll write next to the circled word "Because kendo doesn't use a ball."
Usually students find this to be a fun exercise, so I give them about 6 examples in steadily increasing difficulty.
Once they've done enough examples, I break them into groups and have them come up with their own. In a higher-level or smaller class you could have them do it in pairs or individually. Once they've had enough time to think of several examples, they could either pose a question to the entire class and have them answer for team points, or go around the class and quiz the classmates who weren't in their group.
I find that the "because" sentence is something that seems simple to a teacher but is surprisingly difficult for students to produce, at least at the junior high school level. Try and get them to explain their reasons in English as best they can!