Bridge Builder

Bridge Builder is the classic truss bridge game, drawn as a warm cartoon side view. Every challenge is a gap to span, a meadow brook, a river with a sailing boat, a busy harbour, a red canyon at sunset, with red anchor bolts waiting on the banks. Drag from an anchor or a joint to lay a beam: it snaps to the grid, and a ghost shows where it will land, how long it is and what it costs. Road carries the vehicle, wood is cheap but weak, steel is strong but pricey, and cable can only pull. The secret, as every engineer knows, is triangles.

Press Test and the physics runs, a fixed-step simulation that gives the same result on every device. A truck rolls on, later a bus, a tractor towing a trailer of hay bales and two trucks at once. The bridge sags under the weight and every beam glows from green to yellow to red as the stress rises; push one too far and it snaps. If the deck fails, the vehicle drops into the water with a splash and bobs there, nobody hurt, and Back puts your bridge back exactly as it was. Forty brain-teasing challenges grow from a six-square creek to a thirty-square canyon, with stone piers you may lean on, power lines you must stay under and boat channels you must keep clear. Each challenge awards three stars: get across, stay under budget, and keep every beam below 70 percent stress. Every one has been proven solvable within its budget.

Free build is the place to relax: a long valley with no budget, a rocky pier in the middle and two tall towers for cables. Send the truck, the bus, the tractor or two trucks over whatever you dream up, keep up to twenty bridges in your browser and press Photo to save a picture of your favourite. Undo and redo go fifty steps back, the eraser tidies up, and the zoom buttons, the mouse wheel or a pinch get you close to the joints. Keys 1 to 3 pick wood, steel and cable, 4 picks road, Space tests, Z and Y undo and redo, P pauses and M mutes.