how to list sub-pages of active page

I want to list a sub-pages of example page when it is active
-home
-about
-example
-portfolio
-contanct

This is normal.
And after click example link i want to have this efect:
-home
-about
-example (active)
–example menu 1
–example menu 2
–example menu 3
-portfolio
-contanct