I've always had a little trouble getting my head around this third law, the idea that every action has an equal and opposite reaction. I try to keep two examples in mind:
A book lying on a table is pushing down on the table, because it's being pulled downward by gravity. The table is pushing back, and its capacity for pushing back to the same extent that the book is pushing on it is what keeps the book from crashing through the table.
That example is everyday, but it's not very dynamic. Here's the second example that I like to keep in mind:
If an astronaut in space, just floating there, throws something forward -- say, a spare screwdriver -- the equal and opposing force will propel the astronaut backward. The force of the backward movement of the astronaut will equal the force of the forward throw of the screwdriver. (Why an astronaut would carry a spare screwdrive in space is another problem altogether!)
Those examples have always helped me. The website below, written for students, gives more discussion and examples.
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