The “Walking Sphere” is a simple design that is so satisfying to play with. A gentle tap is enough to get it to roll. It’s not magic or anti-gravity—it's smart engineering that uses weight and balance to make gravity work in a surprising way.

Nested discs can look like they are rolling uphill, but it’s really a physics trick. When two discs are crossed at right angles and their weight is placed just right, the center of mass (the balance point of the object) stays at the same height as the discs roll. Objects naturally move in ways that lower their center of mass, so this design lets the discs move while still following the rules of gravity.

As the discs roll, one part of the object goes up while another part goes down, like a seesaw. Even though the whole object looks like it’s climbing, its balance point is actually staying level or moving lower. That’s why it keeps rolling “uphill.”

*Edit on Dec. 21, 2025: I just found out that this shape is called an OLOID. Where I “discovered” it naturally with my students, it has existed in physics for a long time. I love how things like this happen from time to time.

A few printing things:

I used a Bambu PLA Red and Amolen Fast Change Silk filament for the Walking Spheres in the pictures. Both printed perfectly, but the fast color changed added a bit of a “WOW Factor” to it.

The first plate is the whole toy. If you let Bambu Studio arrange it automatically, it will save a little bit of filament and time, but in my experience, it didn't print as nicely. It had rough edges and some warping. Leaving it in the vertical position with supports is how I got the best results. 

The second plate has the toy split in half with a peg to help with alignment, and it is “Print by Object”. I've included this version in case you wanted to color each half something different to make it more festive. I did find that when I printed this version with regular PLA (the red in the picture), the peg fit perfectly, but when I printed it in the fast color change silk I had to sand it down a little to make it fit. The peg isn't required, it's jus to help with alignment. 

Disclaimer: This object may contain small parts that pose a choking hazard. It is not recommended for young children or for individuals who may put non-food items in their mouth.