Boltnest

A workbench, a row of bolts, and a mess of coloured nuts stacked in the wrong order.

Sorting them is the whole game and it is harder than it sounds, because a nut only moves onto its own colour or onto an empty bolt, and there are never as many empty bolts as you want.

Tap a bolt and its top run of matching nuts lifts. Tap another and they drop. Get a bolt down to one colour, filled, and it unscrews itself and takes itself off the board.

• Every level is proved solvable before you ever see it. There is no such thing as an unwinnable board here, and the same search that proves it also measures how hard it is — which is how the difficulty climbs smoothly instead of spiking on a bad shuffle.
• The nuts underneath start face down and turn over when they reach the top, so a board cannot be solved on paper before you touch it.
• Tap–tap placement, built for one thumb on a bus.

Plays in the browser. Nothing to install, nothing to sign up for.