How do i step into a method ?( show the source code )

I tried to search for this topic, but all i have found are questions regarding stepping into a method while debugging. Well i just want to step into the method ( show the source code , library, whatever its called ) during normal editing not runtime. Smart step into and almost everythign else is greyed out inside the "Run" tab. I didnt start the debugger or anythign else. Its just greyed out. In my class, whenever my teacher uses a method or Class ( for example String.substring()) he seems to be able to just show this method within a simple keystroke. I cannot figure out how he is doing that..... Pls help me