I was confused about 무엇이 vs 뭐. Turns out 무어시 is preferred for written and formal use whereas 뭐 is better for spoken use.

Inside is not accepted (vs in)?

So, why is it not 속에? I always thought of 안에 as in a room or building and 속에 as inside a box or dresser or whatnot. Is there a more subtle difference?

So 가벼운 means light as in not heavy? Or light as in light rays?

Why isn't it "what is inside this light box"

Could you mark it somehow that this describes weight?