Kidney Beans- Kidney? (Jinzo-o) Really?

I just walked in the door from teaching, laughing. My last group tonight was a class of 3 nine and ten year olds. I was trying to teach them about time, 1 o'clock, 1:15, 1;30. You get the idea. So we played "time bingo" to learn them all. As I ran out the door earlier this afternoon I realized Joey had taken our bingo pieces (a bag of buttons!). So I quickly searched around the house for something to use and decided on the kidney beans in the cupboard. I remember when I was growing up we used dry beans for all sorts of things, including bingo. Well, I must have forgotten I was teaching Japanese kids because they thought it was so funny! First they wanted to know if they could eat them. Dry! Yuck! One girl decided to try it out anyway and finally bit through it about 2 minutes later. And then they asked if they were adzuki beans(which are about the only beans in Japan, those and soybeans), I said "no, kidney beans" and looked up kidney in my dictionary (jinzo-o). I told them they were kidney shaped (which I think is why they're called kidney, I don't know, I've never really thought about it!) and they laughed and laughed. and kept laughing. we didn't get much english in those last 5 minutes of class, but it was funny!