This is the definition of Brainstorming from the Wikipedia:
“Brainstorming is a group creativity technique by which efforts are made to find a conclusion for a specific problem by gathering a list of ideas spontaneously contributed by its members.
In other words, brainstorming is a situation where a group of people meet to generate new ideas and solutions around a specific domain of interest by removing inhibitions. People are able to think more freely and they suggest as many spontaneous new ideas as possible. All the ideas are noted down without criticism and after the brainstorming session the ideas are evaluated.”
In the quote above, the emphasis on “without criticism” is mine. As we know, it is not a natural tendency for people to avoid making criticism. They may even be able to refrain from making an open criticism, but in general it is quite hard for people to identify with the innovative ideas being proposed by others.
Therefore, even if during the brainstorming session the ideas are not criticized, most of them will be immediately discarded when “after the brainstorming session the ideas are evaluated.”
This is very well depicted in the cartoon below: