Out on the inland plains, a roly-poly is a tumbleweed. At the end of summer the dry bush snaps off at the root, and the wind rolls it away across the gibbers, a round tangle of stems that bounces along until a fence stops it.

It's also the pill bug in the garden, the slater that curls itself into a ball when you pick it up. A yoyo is a ball that rolls, so we took both: the name from the plain and the mark from the bug.

Roly-poly is a doubled-up word, the kind Australian English is full of: willy-willy, ning-nong, hooley-dooley. It's fun to say, and for a toy that matters.

The mark is the bug curled up tight, its shell in curved bands, with one eye and two antennae poking out. It's lasered into the floor of each cup, so every Roly Poly has one curled up inside.

Opal starts like any anodised part. It's machined, bead-blasted, then hung in an acid bath with current running through it until a layer of porous oxide grows on the surface.

Until those pores are sealed, they take dye. A solid colourway goes into one tank. A splash gets a base colour first, then more dye thrown at it by hand, one colour after another, each landing where it lands.

Opal is deep blue, then teal, green, violet, orange and pink. Sealing in hot water closes the pores and locks it all in.

No two come out the same, which is the point. The colour lives in flashes, like the black opal at Lightning Ridge, and you see more of it when it's moving.

A 3D-printed copy of the Frogmouth's shape weighs about 32 grams. It looks right and feels like nothing, and the spin is gone in seconds.

So the printed version keeps every surface you play on, the catch zone, the rims and the bearing seat, and changes only what's inside the cup. The cup gets narrower, which moves plastic out into the rims, until a solid PETG print weighs 66.0 g, the same as the aluminium.

A script finds the spot. It moves the cup lip in until the weight comes out right: for the Frogmouth that's a lip at 17.35 mm instead of 24.97. The print ends up with 95% of the metal yoyo's spin inertia.

Every model gets one. Print two halves, add a bearing, pads and an axle, and throw it for a week before you buy the metal one.

Most of how a yoyo feels comes down to where its weight sits. The Frogmouth is drawn so that 89% of its spin inertia lives in the outer 30% of its radius.

The profile is a handful of curves: a flat wall beside the pads, a catch zone that opens out in one long curve to the rim, a rounded rim, and a deep cup that follows the catch zone 2.4 mm behind it.

Then the cup lip moves. Push it out and the rim thins and the yoyo gets lighter. Pull it in and it gets heavier. The same script finds the lip where the whole yoyo, bearing and axle included, weighs 66.0 g.

The other three come out of the same script, so a Mopoke, a Bogong and a Thornbill are each a few numbers away from the Frogmouth.