If there were just one thing that I wish students in my university courses would remember and carry with them into their lives, it is this:
Your perceptions are NOT Reality! Question them always!
The "reality" that each one of us experiences in all moments of our lives is actually a construction of our minds. It consists both of the stimulation that our senses receive (patterns of light, sound waves, pressure and temperature on the skin, chemical molecules in the air and on our tongues, etc) AND our interpretations on this onslaught of information.
This is where things get really interesting because these interpretations are highly edited based on our unique past experiences, beliefs about of the present situation, and our expectations about what is about to happen next. Even fluctuations in our moods, levels of arousal, attention, distraction and so on have a profound impact on what we perceive. This is why we each one of us perceives the same people, objects, and events differently. It is also why our own experiences of the very same things change over time.
Here is a fun experiment to try: as you go through your day, try occasionally reminding yourself that what you are experiencing may not actually be precisely what is happening. How does this little reminder influence your experiences, the assumptions that you draw, and your interactions with others?