It refers to the law that every action has an equal reaction either immediately or at some point in the future.

"To every action there is always an equal and opposite reaction."

"For every action there's an opposite and equal reaction, you know."

Make sure you tell him every action has an equal, opposite reaction.

It's basic science: for every action, there's an equal and opposite reaction.

There's an old natural law that says for every action there is an equal and opposite reaction.

For every action there is an equal and opposite reaction.

But, to every action there is an equal and opposite reaction.

But might history one day identify an equal and opposite reaction going on, too?

For every action, there is an equal and opposite reaction: imagine how different our lives could be.