The idea of equality. The problem of "chicken and rabbit in the same cage" is a very interesting ancient mathematical problem. It seems simple, but it carries a lot of mathematical thinking methods-enumeration, hypothesis, change and invariance (equal thinking). However, as for "chickens and rabbits in the same cage", people just stay on "chickens and rabbits". If you change to other problems of "chicken and rabbit in the same cage", you will not find "chicken and rabbit"