Each DDD offers 24 different learning opportunities in the given topic: a web page or other link, an image, a sound, or a video. 

If you are logged in, the system will hold your place so that doors will remain in the same location and those you have already opened will remain opened, to hold your place.  You can open one door each day, open all 24 in one day, or anywhere in between.  ENJOY!  :O>