Crawlin' man

Ari can crawl and already our lives are changed.
It's amazing to watch how a baby learns to do stuff. For weeks and weeks Ari struggled with crawling, but kept collapsing on his tummy, arms and legs a-flailin'.
Then he made his first tentative steps and can now crawl across a room. He's no commando yet, but once he finds his momentum there will be no stopping him.
This has introduced another fun activity to my already busy day - saying "NO!" a hundred times as Ari tries to play with his new favourite things - power cords.
Particularly enticing is our blue ADSL cable.
I don't really want to cage things off in our house, so I'm trying to drum into the boy that no means no and there are things he's not allowed to touch.
Every day we start again and go through the same procedure. When do babies start to learn stuff like that?