Hi, In the previous exercises, I see list: cc=[[],[],[]] Then, I can have this: ccc=[[[]],[[]],[[]]] I can also have ccc[0] Out[158]: [[]] ccc[0]='o' ccc Out[163]: ['o', [[]], [[]]] I have question: Is there any difference between [[]] and []? [[]] can have deeper assignment and use than ccc[0]='o' Thanks,