Before asking preschoolers to participate in activities where they have to brainstorm, take an activity time to introduce the concept of a brainstorm. Explain that a brainstorm is when you take all of the ideas in your head and let them out, kind of like how a cloud lets out all of the rain during a storm. To reinforce the concept, you can make a craft with cotton ball clouds and big pieces of glitter for raindrops coming out of the clouds.
Start brainstorming with concrete ideas, which are often easier for preschoolers to grasp. Play a categories game by having the kids sit in a circle and take turns brainstorming items in the announced category. For example, if you say "animals," go around the circle and have each child name an animal. Kids cannot repeat a response that another child gave. Go around the circle more than once if kids seem to have more ideas in that category. Other potential categories include fruits, vegetables, colors, items of a specific color, creatures that swim, musical instruments and things with wheels.
Move brainstorming into the abstract by having kids brainstorm answers to hypothetical questions. For example, you could ask them what a dog might be thinking while he sits at home or what might happen if people could fly. Write down the answers so you can read them back to the kids when they are done. Rather than going around a circle and putting pressure on kids to think of something new, have kids raise their hands to share answers. If a child is quiet, call on him early in a round before too many obvious answers have been said.
Teach kids that they should share anything they can think of in a brainstorm, even if it does not seem like the best answer, by having them share the silliest answers they can think of to some questions. For example, ask the kids to brainstorm the silliest way to get from one end of the room to another. Turn it into a physical activity by having them demonstrate their silly methods, too. When kids start running out of ideas, ask "Who can think of something even sillier?" to prompt more responses.