We discovered a new game, and my daughter is obsessed. I push a bottle into one end of a massage roller, and it slides out the other, landing in front of her. She soon joins in, picking up the bottle and pushing it back through. I lift the roller, give it a spin to change the bottle’s direction, and let it drop before her again. She giggles.

After a few rounds, she grabs the roller and peers inside. From the other end, I keep taking the bottle and returning it. I also flick the bottle, making it jump towards her inside the tube. She squeals with laughter.

It got me thinking: maybe tube shapes are uncommon for babies, both now and for babies in prehistoric times. Topologically speaking (that branch of math about counting holes), most toys and dishes she sees are basically discs. Only a few, like instant noodle bowls with handles, are topologically donuts.

To her, a disc is closed. Things just pass by. But with a donut shape, things can move two ways: passing by or going through the hole. Going through, to her, means interacting – entering and exiting. The tube’s larger internal space makes this even more apparent.

The bottle disappearing and reappearing probably seems magical. After grabbing the roller, she might’ve sensed the magic of this shape with a hole.

Okay, full disclosure: I’m probably overthinking the math. Maybe she just likes the bottle, hahaha.