Newton's Third Law of Motion
Newton's Third Law of Motion tells us that
For every action there is an equal and opposite reaction.
Everything moves relative to everything else. If you push somebody, the push forces her backwards, but it also forces you backwards - that's the equal and opposite reaction. Sometimes the thing you're pushing against is so big that you don't notice the reaction (like if you are pushing off from the ground, the Earth is too big for you to see that the Earth is also moving away from you), but the reaction is still there.
Bibliography and further reading about Newton's Laws:
Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University
Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.
More about Professor Carr's work on the Portland State University website
Today's special find from Amazon:
This is a great kit: mold your own human skeleton, put it together, attach the magnets and stick it to your fridge! Learn what's inside your body.
Quatr.us began in 1995 as a student project funded by Portland State University. For the last fifteen years, Quatr.us (formerly "History for Kids") has been entirely independent of the University, using ads to keep the service free.
Quatr.us now has about 3000 articles, all researched and written inhouse by university professors; we try to add a new article every day. About 30,000 people a day visit Quatr.us (that's about a million people a month!), from every country in the world. Our many awards include the Encyclopedia Britannica's Best of Web 2009.