My Take
This is an amped up version of the old encyclopedia. Anything you would look up or research, can be done here. There are two terrific aspects of this site. One - you can change the reading level at any time form primary, intermediate to secondary. Two - there are multiple media types for each subject (most subjects.) You can assign students a topic and they can collect information from an article (which can be voiced), as well as viewing a video and possibly a timeline.
Occasionally, I will run into a topic not covered on the website.
How I Use It
This is a great learning and research tool. It is safe for students to search themselves and collect information in different grade levels. The level of reading is adjustable, which will also adjust the amount of complexity and content of the material. The most powerful tool is the multimedia options. There is often a video, timeline, diagrams or pictures provided with the topic.
This makes for comprehensive short research projects, sub plans to reinforce studying of a subject, or just another science resource.
The only negative is that it is not free.