Brainstorming is the "Imagine" step of the engineering design process — generating as many solution ideas as possible before choosing one to build. The most important rule of brainstorming is to separate idea generation from idea evaluation: first come up with many ideas without judging them, then go back and pick the best ones. Quantity leads to quality because unusual or "silly" ideas often spark practical solutions that nobody would have thought of otherwise. After brainstorming, engineers evaluate their ideas against the problem's constraints to select the most promising one to prototype.
Give students a simple challenge (design a device to move a tennis ball across a table without touching it) and set a timer for five minutes of silent brainstorming where every idea gets written on a sticky note — no discussion, no judgment. Then share ideas as a group and notice how one person's wild idea can improve another person's practical idea. Practice evaluating ideas against constraints: which ideas fit the budget? Which use available materials? Which are safe? The contrast between "all ideas welcome" brainstorming and "let's pick the best one" evaluation teaches the two-phase structure.
You have identified a problem and you understand it well. Now what? You need ideas — lots of them. This is brainstorming, and it has one golden rule: come up with ideas first, judge them later. These two activities — generating and evaluating — use different parts of your brain, and trying to do both at once makes you worse at each.
Here is how brainstorming works in practice. Set a timer (five or ten minutes), and write down every idea you can think of, no matter how wild or impractical it seems. If the problem is "design a way to carry books and a water bottle to school," your list might include: a backpack with special pockets, a rolling cart, a belt with clips, a trained dog that carries your stuff, a robot wagon, a shoulder strap with bottle holder, a vest with book pouches. Some of those are practical. Some are absurd. That is exactly the point.
Why allow absurd ideas? Because wild ideas spark practical ones. "Trained dog" might make you think of wheels, which leads you to a rolling book bag you had not considered. "Vest with book pouches" might seem funny, but it could inspire a backpack design with better weight distribution. If you had dismissed those ideas the moment they appeared, the practical follow-up ideas would never have been born. Creativity is a chain reaction, and censoring the early links kills the later ones.
After brainstorming, you switch modes completely. Now you evaluate. Look at each idea and ask: Does it solve the problem? Does it fit the constraints (budget, materials, size, safety)? Is it something you can actually build and test? This is where you narrow dozens of ideas down to one or two worth prototyping. The evaluation phase is just as important as the generation phase — you need both wild creativity and practical judgment, just not at the same time.
One more thing: brainstorming works well in teams, but only if everyone feels safe sharing ideas. If one person makes fun of another's suggestion, people stop contributing. The best brainstorming teams follow the rule strictly — every idea gets written down, and nobody says "that won't work" until the brainstorming timer goes off.