As an instructor, I have great difficulty teaching Newton's 3rd law: "For every action there's an opposite and equal reaction." This is very basic and very old physics but it's hard to teach. A typical example of the false reaction force answer is the following:

I hold an apple in my hand. The earth pulls down on the apple with a gravitational force. What is the reaction force to this?

The answer most of them will give is that the reaction force is "my hand pushing up on the apple." Arggghhhh! Of course the reaction force is "the apple pulling up on the earth."

Students fail to realize that the opposite and equal reactions have to be between the same pair of objects. That is, forces arise as pairs. I wish they'd just rename the law so that it makes it more clear that the reaction force has to operate between the same pair of objects.

I demonstrate the law by holding a long spring in my hands and telling them that forces are like this spring. When it applies a force on one end it applies a force on the other (assume massless spring). Next quarter I'm going to try some more extreme measures on this, clearly I'm failing.