The male organ i.e. penile structure is not made up of bones but contain strong muscles that eat up more blood and retain to stand up and remain erect while the man is aroused s*x.ually.
Said this,when the man is se.xually excited seeing na.ked woman or duringthe for.eplay, the muscles of the penile structure retains the blood and make the organ erected. The best hand exercise to make p.enis bigger based on this is stretching it naturally.
During the day, stretch the organ downward (when it is not erected of course) a few times. This naturally lengthens the structure and you get bigger organ.
