Question

when common salt is applied water comes out from the cut pieces of onion and cucumber why

Answer 1

The process that causes this is known as osmosis - in this specific case it is exosmosis.

So what happens?

The process of osmosis is when water molecules move from a region (solution) of low concentration to a region(solution) of higher concentration through a semi-permeable membrane.

When salt is applied to cucumber or cut onion pieces, it makes their surrounding more concentrated than the concentration of their cell sap. This then causes the water molecules in the cells of the cucumber and the cut onion pieces to move to the surrounding through their cell membrane( which is semipermeable). 

This is the water that is seen in coming out of the cut pieces and cucumber. 

Answer 2

By the process called as Osmosis , water comes out from the cut pieces of onion and cucumber When common salt is applied.

Explanation:
Osmosis is the passage of water from a region of higher water concentration through a semi permeable membrane to a region of low water concentration.

When salt is added to onion and cucumber cut pieces, the presence of salt makes the solution hyper tonic environment ,[ means surrounding region is more  salt concentrated than the  concentration of cell.]This makes the water molecules from the cucumber and onion cell to move to surrounding through their semipermeable cell membrane.