It’s painful to walk on small pebbles with bare feet. Why does a person not feel pain if he walks on the same stones in the water?

Because in water, in addition to gravity, a pushing force directed upwards acts on a person.

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.