Why does an iron nail sink in water, but a multi-ton ship not?

The nail is made entirely of iron, and there are no cavities with air in it. The ship does not sink because part of it is located underwater, and almost all of it is filled with a substance lighter than water (usually air).

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.