Now, let's teach grade two students. If physics class has just started, it is difficult for students to understand buoyancy, and refraction is simpler, but it is not interesting. There are many experiments about buoyancy, which are very interesting.

It doesn't matter which class you are familiar with, only which class you can better cooperate with. You must choose an energetic and active class, otherwise the open class will be lifeless and embarrassing. This is my answer.