http://inventionatplay.org/playhouse_main.html
http://www.mos.org/sln/Leonardo/GadgetAnatomy.html
http://www.mos.org/sln/Leonardo/InventorsToolbox.html
http://www.mos.org/sln/Leonardo/LeosMysteriousMachinery.html
http://www.kent.wednet.edu/staff/trobinso/physicspages/PhysicsOf/Wade/physics.htm
http://www.edheads.org/activities/simple-machines/sm-glossary.htm
http://www.fi.edu/qa97/spotlight3/spotlight3.html