Alex went to visit his friend Fred. When Alex arrived at Fred's apartment, there was a piano stuck in Fred's doorway. Fred was inside, struggling with the piano. "I'm afraid I've gotten this piano stuck in the doorway," Fred lamented.
"Let me help you with it," said Alex.
"Good," replied Fred, "I need all the help I can get."
The two of them struggled for several minutes. But the piano never budged!
"Fred, I think we're never going to get this piano into your apartment," sighed Alex.
"Into my apartment?!?!" yelled Fred, "I've been trying to push it out of my apartment!"
( groan from reader! )
The point of this ancient joke is simply to remind you that the direction of a force is important.
Force is a vector!