Many tools are available on internet for downloading you tube video. But if you download you tube video without any tool this tips is for you.
Open YouTube in safari browser and select your favorite video, press Ctrl + Alt + A or click window in menu bar and click on activity option. Search biggest size link (normally in Mb) and double click this link.

New window will be open and it asks you for saving file.
If safari don’t ask you for saving file just copy all link from your address bar and open with other browser like Firefox , Google chrome , Internet explorer, opera , and press Ctrl + s and save video file.
You can play this file with popular free VLC player

