Science fair advice tends to focus on presentation: clean boards, rehearsed openings, confident delivery. Those help. They are also not what separates projects at higher levels of competition.
They test whether you understand your own method
The most common judging move is to ask why you made a specific choice. Why this model, why this sample size, why this control. A student who can explain the reasoning, including the tradeoffs, is immediately distinguishable from one who followed a recipe.
The safest preparation is to write down every decision you made and your justification for it. If a justification is weak, you have found the question you will be asked.
They probe the limits
Judges ask what would break your conclusion. Saying that nothing would break it is the wrong answer. Naming the specific conditions under which your result would not hold demonstrates that you understand the boundaries of your work.
They notice when you overclaim
Stated impact should match demonstrated evidence. A simulation result is a simulation result, and describing it as validated field performance invites exactly the follow-up you do not want. Precise, modest claims survive scrutiny that inflated ones do not.
The pattern across all of it is simple. Judges are evaluating how you think, and a smaller project explained rigorously outperforms an ambitious one explained loosely.