Resist correcting anyone during the six images. The point is not to sort them, it is to get learners to notice they are already using a rule they cannot name. Ask: "What rule are you using to decide?"

The star is the one that exposes the most interesting confusion. Learners who call it a planet are reasoning from 'round bright object in space', which is exactly the reasoning the IAU rejected. Ask what the difference is between something that makes its own light and something that only reflects it.

The Moon will split the room, and that is the most useful disagreement of the session, so do not resolve it. Take both sides, write the arguments up, and leave it hanging. Come back to it after the pathways.

Ask directly: what would happen to Earth if it stopped moving sideways tomorrow? The answer, that it would fall straight in, is what makes orbit make sense.

Circulate with one question ready: "if you discovered a new object tomorrow, how would you decide what to call it?" It primes the activity and it tells you immediately who has grasped that classification is a decision rather than a fact.

Cover the Earth slide properly before moving on. Say all three rules out loud against the one object everybody already agrees about. Without that model, learners name the object instead of classifying it, and naming is not the skill you are after.

Insist that every verdict states which rule fails, not just yes or no. "Not a planet" is worth nothing. "Not a planet, because it orbits a planet rather than a star" is better. Where an object passes, encourage recitation of all three.

Rule 3 is where the errors will be. Learners read 'cleared its path' as 'nothing else is anywhere near it', which is not what it means and would disqualify most of the solar system. Say early that it means the object is the dominant thing in its lane, not that the lane is empty.

Part 2 asks what changed. Expect "Pluto changed" or "the scientists were wrong". Both are worth hearing before you push. Pluto is exactly as it was in 1930. What changed is that we found thousands of similar objects and had to decide what that meant.

The lock-in quote is Alan Stern, who leads the New Horizons mission and is the most prominent scientific opponent of the definition this session teaches. He still argues Pluto is a planet. Saying so is a stronger ending than the quote alone, because it shows the argument is live.