"Let's Play Quantum Mechanics" is the 26th episode of the fourth season of Kaeloo, and 176th of the series overall.
Mr. Cat wakes up one morning in a great mood and buys some chips, a can of soda, and a newspaper from the local store. Just as he’s about to sit down for a relaxing morning, Stumpy swings in on a rope while screaming for Mr. Cat. The latter promptly whacks him with a mallet and asks, with a sarcastic smile, why the squirrel wants to ruin his morning. Stumpy asks Mr. Cat to explain quantum mechanics to him. Mr. Cat is initially reluctant, but Kaeloo and Quack Quack, hiding behind a bush, encourage Stumpy to resort to flattery. Stumpy points out that Mr. Cat knows everything and is very intelligent, and despite noticing the obvious attempt to flatter him, Mr. Cat is pleased and agrees.
Quack Quack is offended that Kaeloo and Stumpy didn’t ask him first, but Kaeloo explains that the way he explains it is too complicated. Mr. Cat says quantum physics is “very simple”, but even his explanation of what the subject entails is too complicated for his friends to understand. He decides to give them a demonstration of how it works so they can understand it better. He asks for a volunteer and Stumpy offers, but Quack Quack is chosen instead.
Mr. Cat puts Quack Quack inside a giant container and tells his friends he’s going to replicate the “Schrödinger’s duck” experiment, which involves repeatedly killing Quack Quack and having him come back to life. He gets carried away and Kaeloo tries to attract his attention, but he ignores her until he finds out that Quack Quack has escaped the container. Kaeloo asks what the point of the demonstration was. Mr. Cat explains that since quantum physics says time doesn’t exist, Quack Quack is both dead and alive at the same time. Mr. Cat and Quack Quack have fun talking about waves and particles but Kaeloo and Stumpy still don’t understand what they’re talking about. They decide to ask Olaf instead.
Olaf complains about how “yeses” and “nos” have been replaced by “maybes” thanks to quantum physics. Kaeloo doesn’t understand, but Stumpy likens it to a time in class when his teacher asked him if he’d done his homework and he replied “maybe”. Olaf hates “maybes” because they encourage imagination, which could distract people from order and discipline. Kaeloo disagrees and shows him her own imagination. Olaf has them locked up in a prison cell for daring to go against him. However, using the principles of quantum mechanics and their imagination, Kaeloo and Stumpy escape from the prison by wishing themselves to be outside it. Kaeloo concludes that quantum mechanics can be defined as “complicated words that can be replaced by magic, all magic”.
Kaeloo Sings A New Song, Through song, Smileyland gets turned into a 2D world where all forms of shapeshifting and cartoon physics are possible. During the song, Kaeloo flirts with Mr. Cat. When the latter responds by talking about how much he loves Bad Kaeloo and dreams of spending his future with the toad, Kaeloo becomes jealous and angrily tells him “don’t count on it!”
In an attempt to anger Kaeloo, Mr. Cat asks her if her butt is “quantic or fake”, which embarrasses her. She transforms and punches him so hard he gets sent into space.
- This is the first time in the series where animated humans appear. However, it is not the first time in the franchise; the Quack Quack XP episode "Experiment 63: Quack Quack's Speed" featured a Brazilian human man.
- The human versions of Kaeloo and Mr. Cat resemble preteens; evidence from other episodes suggests that Kaeloo and Mr. Cat are canonically in this age range.
- This is the second episode where Kaeloo openly and directly expresses her romantic feelings to Mr. Cat, the first being "Let's Play Baby-Sitting". However, in this episode, they are more sincere and not part of the game.
- Mr. Cat says a few words in German during his lecture.