Quite Simply, action = reaction.

In more sophisticated terms:

For every action force, there is a reaction force which is equal in magnitude and opposite in direction.

In other words, if you push on something, it pushes back on you, or if something pushes on you, you push back on it.  Even if you don't want to (it transcends free-will)!

Try the following for more assisstance!

http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_CPworkbook_ActionReaction1.xml

http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_CPworkbook_ActionReaction2.xml