One of the major challenges with brainstorming an idea is that people want to immediately evaluate the idea. It's like playing "whack a mole" where creativity gets squashed with every suggestion.
I was recently working with a team that loves to play "whack-a-mole". While I stated the traditional brainstorming ground rules, I also added one more to prevent this tendency to evaluate every single idea:
"Be curious."
Here's how.