I’ve been playing a lot of Holedown recently.

Screenshot from the game Holedown

It tickles my brain in so many ways. I love particle systems, reflections, and bouncing. (It was fun to introduce Alex to snooker when we were at Archerfield in June. While everyone else was enjoying the loud party, Alex and I snuck off to the snooker room and spent a hilarious couple of hours failing to pot balls.) The paths the balls take through the blocks sometimes have the pleasing symmetry of juggling patterns, and sometimes they devolve into something that looks like chaos, but which is actually a playful visualization of a Dynamical System. (My favourite maths course at university.) And then sometimes the balls will get stuck bouncing back and forth faster and faster in an almost perfectly horizontal or vertical cascade between two parallel walls, similar to the resonating cavity of a laser.

But best of all, they’re a constant reminder of one of my favourite physical objects: the corner cube reflector:

Diagram of a corner reflector
By Chetvorno [CC0], from Wikimedia Commons

Geometrically speaking, they’re so simple and obvious, but I’ve always found their behaviour magical and charming: no matter what angle you choose, any ray you send into it is guaranteed to be reflected back exactly parallel to the original. There are arrays of corner reflectors on the moon. If you shoot a laser at them, its light will bounce straight back at you. A lot of the light is absorbed or deflected along the way, so it’s a tricky experiment, but it allows us to measure the distance between the earth and the moon down to the millimeter. Science!

Holedown is very nicely designed, with lots of subtle animations and effects, and elegant mechanics. It’s simple and addictive, but it’s also really easy to pause and walk away from for a while. As you progress, the game gets harder and more intense, but it doesn’t get faster. You can think about your moves for as long as you like, and resume whenever you want. It doesn’t use microtransactions, either. Just buy it and play. It’s adorable.