Make dice (saikoro) about 10cm x 10cm or larger. Go through the numbers with the students. I explain eleven (7-11) and twelve (difficult) for 5th and 6th graders. I play boys vs. girls in a line. The first girl and boy come forward. They roll the dice, then the teacher calls out a number between 1 and 6. The students must add the dice number with the number the teacher says. The quickest gets a point and the next two students come forward. No hints from team-mates allowed instant penalty. The Japanese teacher keeps score.