I learn english and often see phrase to do nothing, but it's nothing, how possible to do it?

You are asking it from a philosophical point of view.

In plain English, "to do nothing" means to not do anything at all. It describe a lazy person who spends his/her time wasting the day away by not doing anything.

Example:

Somebody who stays in bed and sleeps all day is a lazy person who has nothing to do. He's doing nothing but sleeps.

In your philosophical argument, the act of sleeping IS doing something; therefore, it's impossible that he is not doing anything.

.... If you define the phrase that way then, ok, you do have a point; but that's not what "to do nothing" means in plain, everyday English.

When someone says they're doing nothing they mean they're not doing anything. It's not meant to be taken literally - just that they're not doing anything important enough to mention. If I say I'm doing nothing over the weekend, a person would usually take that to mean that I will just stay home and be lazy.

The best example would be doing something that is embarrassing. Say you are watching a movie. A man, woman, and a bed. They are getting undressed.

Suddenly you heard your grandmother about to walk in. Quickly you switch off the movie.

She asks " What are you doing?"

To save yourself embarrassment you reply " Nothing".

When parents tell their children to clean their room and they forget, mother's usually scream, " you do nothing all day, and yet you still can't tidy up after yourself!"