The amount of person uses his imagination is inversely proportional to the amount of punishment he will receive for using it.
Her implication: It's okay for children but not for adults to spend their time what-iffing. We have been trained to respond to unusual ideas by saying, "That's not practical," instead of, "Hey, that's interesting; I wander where it will lead our thinking." The danger of premature evaluation is that nothing will be conceived. If the people who were discussing the possibility of "gunpowder paint" or "coin-return trash cans" had said, "Be Practical", they never would have given themselves an opportunity to take a germinal idea and turn it into a usable one.
To be sure, you need to be practical for almost all of your daily activities. If you aren't, you probably won't survive very long. You can't live on imaginary food, or stop your car with "what-if brakes" that save brake lining by working only 75% of the time. Being practical is important in the world of action, but practicality alone will not generate new ideas. The logic which works so well in judging and executing ideas may stifle the creative process if it prevents the artist in you from exploring unusual germinal ideas.
s world was built by practical people who knew how to get into a germinal frame of mind, listen to their imaginations, and build on the ideas they found there.