Adding a song to an existing playlist.

I tried dragging a song into an existing playlist, and it wouldn't go. After trying all sorts of work-arounds, I noticed that the playlist had a "gear" icon next to it. It is a smart playlist! So I brought up the playlist on my screen and found that under the title of the playlist it says, in blue type, "Edit Rules." So I went to the edit rules and added the song. By the way, those rules are rules that I MADE then I created the smart playlist. Doh!