At its core, Calvin and Hobbes tends to stay grounded in some semblance of reality. Sure, there’s the pervading question of whether Hobbes is real or make-believe (a question that creator Bill Watterson refuses to answer). But most stories take place in the real world as Calvin experiences it.

While Calvin has his solo adventures as Spaceman Spiff, those fantasies don't usually involve Hobbes. Sometimes, however, the titular pair of best friends will find themselves in a fantastical situation straight out of a science fiction story. Whether it’s time travel, cloning or journeys through outer space, these adventures make up some of the most memorable that Watterson ever produced. The best of these aren’t limited to a single day’s strip, but unfold over the course of days or weeks. Here are the greatest sci-fi storylines from Calvin and Hobbes’ run.

10 The Transmogrifier’s Debut Turns Calvin into a Tiger

March 23 – April 3, 1987

From Calvin’s perspective, it must seem like Hobbes has the ideal life. With no school, no chores, and no parents to deal with, things are much simpler for tigers. That’s why, when Calvin invents a transmogrifier using a cardboard box, he instantly decides to turn himself into a tiger. It doesn’t turn out quite like he imagines, as he steps out of the box looking like a miniature version of his best friend.

Calvin quickly realizes that being a tiger seems better than it actually is. It doesn’t bring him any closer to Hobbes, as they stare awkwardly at each other before Calvin starts to wonder when they’re going to eat. His mother doesn’t notice the change in him at all. Eventually, he decides to switch back to his regular self. But this discovery of what amazing things can be done with a cardboard box opens up a world of possibilities that Watterson explores throughout the strip’s run.

9 Calvin’s Thinking Cap Really Expands His Mind

November 15 – December 4, 1993

For someone who is so opinionated about things, Calvin has a surprisingly tough time coming up with a topic to debate for a school paper. Instead of thinking of something he’s ionate about, or bouncing ideas off of Hobbes, he decides to invent a thinking cap to enhance his brain. Using a metal colander and his ever-reliable cardboard box, he builds a machine to “conduct electrical brain impulses and reflect brain waves.” It works, and while Calvin has always had a big head (metaphorically speaking), the thinking cap literally grows his head to contain his augmented brain.

After successfully unraveling the secrets of the universe, Calvin finally decides to write a paper arguing that Tyrannosaurs were predators and not scavengers. But as his brain starts to shrink back to its normal size, he spends all his time making drawings of a T. rex eating people at a museum. By the time he’s ready to start writing, it’s time for bed. He quickly cobbles together a one-sentence paper that argues it would be “pretty bogus if they just ate things that were already dead,” which earns him a D on the assignment.

8 Calvin and Hobbes Go Back in Time to Get Rich with Dinosaur Photos

June 25 – July 7, 1990

If there’s one thing Calvin loves more than dinosaurs, it’s coming up with get-rich-quick schemes. He combines both of these when he and Hobbes go back in time to the Jurassic period (about 150 million years ago). Unlike their first journey into the past, when they visit the era of dinosaurs by accident, this time they’re prepared. They’ve even packed snacks, which Hobbes is more interested in than any of the more fantastical stuff going on around them.

They first spot a Diplodocus and snap a few pictures before running into a carnivorous Allosaurus. They’re forced to flee for their lives, and only escape by distracting it with their snacks (specifically, with Calvin’s snacks, since Hobbes still wants his). When they return and get the film developed, Calvin’s parents think he’s just taken pictures of his toy dinosaurs, dashing any hopes that they might profit from their experience.

7 Calvin’s Goody Two-Shoes Clone Drives Him Crazy

March 18 – April 3, 1991

Calvin’s first foray into the world of cloning doesn't exactly go as planned (more on that later). However, he revisits the idea and tinkers with his duplicator, making it so that it will just duplicate his good side rather than his entire personality. The result is a version of Calvin who combs his hair, cleans his room, eats his vegetables and participates in class. This all goes great, until he develops a crush on Susie, offering to carry her books and writing her love notes.

Watterson crafts this story a little differently than most of his narratives. At first, the reader just sees Calvin acting nice, and is likely just as confused as his mother is. Eventually, he clues us in to the true nature of what’s going on. And while Calvin is happy to have a patsy to do his chores and schoolwork, the one thing he won’t stand for is someone ruining his reputation by being nice to a girl. He and the nice Calvin almost come to blows, until the duplicate has an evil thought and pops out of existence.

6 Calvin Brings a Snowman to Life

December 31, 1990 – January 19, 1991

The snowmen that Calvin tends to build are equally grotesque and creative, and have resulted in some of the most beloved strips Watterson ever produced. In this story, he finally gets to experience the horror that others must feel when looking at his creations. After willing a snowman to life, he’s shocked to find that it actually works. But this snowman is no Frosty, and chases after its creator.

While Calvin and Hobbes try to figure out how to kill the creature, it starts building snowmen of its own. It eventually amasses an entire army of “Deranged Mutant Killer Monster Snow Goons.” Under cover of night, he and Hobbes sneak out and spray the snow goons with a hose. This freezes them in their place and ends their threat, while also getting Calvin in trouble for turning the lawn into a giant sheet of ice.

5 Calvin and Hobbes Try to Go to the Future, End Up in the Past

August 31 – September 11, 1987

The first time Calvin uses his cardboard box to travel through time, things go as wrong as they possibly can. He and Hobbes set out to explore the future, hoping to swipe something futuristic and bring it back to the present so they can get rich. However, they don’t notice that they’re facing the wrong way, and end up traveling to the distant past.

First believing that they’re in a futuristic arboretum, they quickly realize their mistake after encountering a dinosaur. They run for their lives – and obviously get into an argument over whose fault this is. They manage to get back to their time machine and return to the present. While this isn’t what they hoped for when setting out, it’s a valuable learning experience that allows them to be more prepared for subsequent trips through time.

4 Calvin Invents a Transmogrifier Gun, Gets Stuck as an Owl

February 8 – 20, 1988

Calvin is quick to give up on things when he encounters challenges (or just gets bored). But he’s also not one to let a good idea go to waste. Returning to his transmogrifier invention, he modifies it from a cardboard box to a gun that he can use to change into whatever he thinks of. However, he makes the mistake of trusting Hobbes with the task of transmogrifying him into something, and is shocked when his buddy turns him into a chicken.

This story is the rare one that continues through a Sunday strip. Most ongoing storylines would pause after Saturday and continue on Monday, with Sunday being an unrelated comic strip.

This leads to one of their classic fights as they struggle over the gun. Watterson gets a chance to show off some of his more cartoony character designs as the two turn each other into a duck, a pig, a pterodactyl, an alligator and a chimp, among other animals. Eventually the gun breaks, leaving Calvin stuck as an owl. He’s thrilled when he realizes he won’t have to go to school anymore, only to be disappointed the next day when he wakes up a human once again.

3 Calvin and Hobbes Travel to Mars

September 12 – October 1, 1988

For a 6-year-old, Calvin has a pretty cynical view of humanity, especially when it comes to how we’ve treated our own planet. Realizing that humans have wrecked the environment and that he’ll grow up to inherit a doomed world, Calvin decides to leave Earth. He and Hobbes blast off into space on their trusty red wagon, making a quick journey to Mars. They think they’re the only ones on the planet, until they encounter a “weirdo from outer space” who already lives there.

After first being freaked out, Calvin and Hobbes realize that Mars is this creature’s home. Once again showing a maturity beyond his years, Calvin concludes that they had better return to Earth and fix their own planet, before ruining someone else’s. While it’s true that he can often be immature and selfish, this is a rare example where Calvin acts thoughtful, and even a bit altruistic.

2 Calvin Invents His First Duplicator

January 8 – February 2, 1990

The first time Calvin tries to duplicate himself with a cardboard box, it creates total chaos. He comes up with the brilliant idea of making a copy of himself to do chores and homework. However, this plan spectacularly backfires when the duplicate decides he doesn’t want to do any of that stuff, either. Clearly, it has not only copied Calvin’s physical body, but his personality as well.

Things get even more out of hand when the duplicate makes duplicates of his own, leading to a swarm of Calvins. Each one gets in trouble at school, partially because a different duplicate attends each day, and partially because they’re just being Calvin. It becomes too much for even the original, who decides to get rid of the duplicates by transmogrifying them into worms.

1 Calvin Travels Through Time to Avoid Homework

May 20 – June 6, 1991

As many of these adventures prove, Calvin will go to just about any length to avoid doing work. When he’s assigned to write a story for class, he decides to skip the hard part and just travel into the future when, theoretically, he’ll be finished writing it. He journeys from 6:30 p.m. to his bedtime at 8:30 p.m., and discovers that he still hasn’t written it yet. The past and future versions of Calvin argue over whose fault this is, before deciding to blame it all on 7:30 p.m. Calvin.

The two Calvins use the time machine to travel to 7:30 and confront the Calvin they believe should have written the story. Meanwhile, the 6:30 and 8:30 versions of Hobbes decide to stay put in 8:30 while the Calvins sort out their mess. They realize that it would have been easier if Calvin had just written the story in the first place, and decide to write it together while they wait.

Taking inspiration from what’s going on, the two tigers write a story titled, “How Hobbes, the Handsome Tiger, Saves the Day…No Thanks to the Time Traveling Chowderhead.” At first, Calvin is outraged, but it ends up earning an A+ from his teacher. Combining time travel, multiple Calvins fighting each other, and Hobbes being the voice of reason, this storyline contains all the elements of a classic Calvin and Hobbes science fiction tale.