Season 3

When Nick and Sally's boat sinks, they need to build another boat, but don't know how. At trip to Floating Island helps them discover that along with needing the right material, you also need to make the shape of your boat float.

Nick is in pursuit of finding out how to build a drawbridge. In Spansylvania, they meet a dragon who's in pursuit of them – and along the way they find out that some materials are better suited for bridges than others.

When Sally destroys Nick's model of a mountain, the Cat takes them to meet Sara of Serendip, an inventor who helps them understand that you can make new discoveries through your mistakes that involves mixing colors and making a volcano.

Nick discovers that Sally can't hear him when he yells from his bedroom window if their bedroom windows are closed. The Cat takes them to the Soundy Sea and they learn that sound travels in waves.

The kids want to get a trunk up into the treehouse and go to the Machine-a-Ma-Zoo where they're introduced to some simple machines, but learn about the pulley.

Sally loses her slidiness and when they go to the Frictionarium, they find out that there's a force called friction that can stop a person from sliding.

Nick and Sally are having a sleepover in the treehouse and Nick is startled by a shadow. The Cat explains that shadows aren't scary if you know what they are. He takes them to Shadowland where they learn about how the position of the light can make shadows grow or shrink.

Nick unintentionally loses Sally's bouncy ball, they go to Ballaballo and learn how different balls are made of different materials that make them bouncier.

Nick and Sally need a strong piece of string to pull their train, but it breaks, and they don't have anything stronger in their craft box. The Cat takes them away, but the Thingmajigger malfunctions and they have to make an unscheduled landing. It turns out they've shrunken and are inside the craft box, where they discover that they can create a stronger string by braiding several strands together.

Nick and Sally are in the treehouse making funny faces at themselves in the mirror when they notice a bird flying back and forth into the tree branches. They discover that the bird is building a nest and they want to get a closer look but can't do it without disturbing the birds. The Cat takes them to a mirror world where they learn how reflections work and they concoct a kind of periscope that allows them to watch the bird without disturbing it.

Nick gets some new train cars for his train set and wants to add them to his train, but the magnets won't connect. The Cat takes them to a magical magnets world where they learn how poles attract or repel.

Nick is trying to learn how to ride his bike and in Balencia, he discovers the things to keep in mind when you are trying to maintain your balance.

Sally wants to fly her kite but can't seem to work out the timing to get the wind right – you can't see the wind. The Cat takes them to Windnasium where they learn to see wind in all kinds of ways.

When Nick's favorite flyer gets stuck in the tree, Nick wishes he could jump really high and retrieve it, but gravity holds him down. Cat takes them to Gravity Drop where they discover what happens when there is more or less gravity. Nick figures out how to use gravity to get his plane back.

Nick wants to create a new home for the cricket in their backyard, but the cricket won't go in. The Cat takes them on a journey but has to make an emergency landing on Lagoona-Maroon where they discover what they really need when it comes to making a shelter.

Nick and Sally are frustrated by a puddle that always forms at an inconvenient spot on the sidewalk. They go to a water park and learn that water always seeks the lowest level.

Sally is trying to make a vase for her mother but each one she tries falls over. The Cat takes them to Blueprintia and they discover what they can learn by designing before building.

The kids are creating a game that involves "fishing" for prizes, but they can't "catch" any of the prizes. The Cat takes them to an arcade where they discover what will (and won't) be attracted to a magnet.

The kids need to move a big rock in their backyard so they can plant an apple tree. The Cat takes them to Machine-a-Ma-Zoo where they learn about levers.

Sally is playing a song on an instrument made up of different things in the Treehouse but she can't find the right note for her song. In Gongolia, they find out that bigger things make lower sounds and smaller things make higher sounds.

Sally keeps getting found when they play Hide and Seek. Cat takes them to Tagaloo to meet a great seeker. They keep trying to catch up to the elusive Tagaloop, and along the way they find out that they can use more than their sense of sight to find things.

Sally loses her special bead in the sandbox and they can't find it. The Cat takes them to the Hole-Lot-of Funhouse, where they have to make their way across different floors without falling through. Along the way they discover how filtering works.

When Nick's magic trick causes his toy car to disappear under the porch the Cat takes them to Arcade Island to win a Gasper-ma-clasper. But instead they find out the magic of bumping things around.

The kids want to make a great snowman but the clothes they have chosen are frozen in the ice and snow. Cat takes them to Coldsnap Island where they discover that when they warm up ice or snow it becomes water.

Nick and Sally can't agree on the "right" way to sort their toys. The Cat takes them to Toborrowland to help him return a bunch of things. As they confront different obstacles, they have to come up with different ways of sorting Cat's things, and they realize there are lots of ways to sort.

Nick and Sally try to save a bubble for Sally's mom, but they always pop. In Odds-n-Endsville they find out that landing bubbles on some surfaces helps the bubbles last longer.

When Nick's water glass suddenly falls over without anyone touching it, Cat comes to help them solve the mystery. They go to The Great Causeway and discover the world of cause and effect.

Nick and Sally are playing musical instruments and notice that Fish's water is rippling. Cat takes them to the Jingly Jungle for a concert by the great Vibrato. They discover that sound and vibration go together.

Nick and Sally's soccer game is obstructed by a large log that they can't lift out of the way. In Machine-a-ma-zoo, they discover that there is a much easier way to move that log than by lifting it.

Nick's new plans is flying well but is a little dull so the kids add buttons and glitter but then they don't understand why the plane isn't flying as well as it did. In Odds n' Endsville, they discover that there can be too much of a good thing, and sometimes keeping it simple is the best solution.

Sally thinks that the three stick she has are not enough to make anything. Cat takes them to a place where they can get more sticks. Along the way, they discover just how many things you can do with three sticks.

When Nick drops the mail bag into his train set, his fingers are too big get it out. Cat shrinkamadoodles them so they are small enough to be in the train set. But when the Thingamajigger falls out of reach, they need the right sized tool to help them out.

Sally is convinced that a tall thin glass of juice holds more than a short wide glass so Cat takes them to Point O'View where they learn that getting evidence can cause them to change their mind.

Nick and Sally wonder if they can walk on a cloud, and the Cat takes them to Mount Knowmore. As they climb up, they discover hard surfaces and watery surfaces and walk through fog. And when they get to the top, they know more about walking on clouds.

Sally invents a secret treehouse handshake, but Nick can't remember the sequence. Cat takes them to Over-and-Overton, where they figure out how to get through a series of obstacles by recognizing patterns.

When Nick and Sally don't understand why one toy car goes faster than another, the Cat needs their help to move some big pictures. Along the way, they find out about how to make things more aerodynamic.

Nick and Sally are disappointed when their toy's battery runs out of power. The Cat takes them to Lots-o-Lakes Land to see the flight of the Dipsydoodlers. Along the way, the Thingamajigger runs out of Go-Go-Go juice and they have to find other ways to make it go.

Sally and Nick have wrapped Father's Day gifts, but when they get mixed up the kids can't tell which is which. Cat takes them to Zippy Zaroo's Cookie Factory where each cookie has a toy inside. When the shrunken Thingamajigger ends up inside a cookie, they have to figure out which cookie it is in.

Sally wants to make a perfect mobile with four identical rubber ducks, but she has only three ducks. The Cat takes them to The Bazaar at Balancia Point, where everything is in balance and has to remain so. Sally finds out there is more than one way to balance things.

Nick and Sally are trying to build a sandcastle but the sand is either too dry or too wet. The Cat takes them to see the Song Flower Sisters, who need just the right mixture of water and soil before they will sing.