It is impossible to keep up with the huge number of internet sites describing how things work. 
Here are some of the sites.

We would greatly appreciate any other links to similar information.
If you can supply such information, please help us.

National Science Teachers' Association - a big list

The MadSci Network - Answers to your questions (or at least somebody's questions)

Scientific American - Answers to Science Questions

Physlink.com - more ask the expert answers

How Things Work - from The University of Virginia

How Stuff Works

Exploratorium Science Snacks - Simple science and learning experiments

Explore Science - This is a commercial company developing web based content

Periodic Table from chemicool

Web Elements - A very informative periodic table

U.S. Naval Observatory - Lots of information on sun, moon, and stars

Space Shuttle Photographs

Visible satellites and times - Heavens Above