How to play
Tap a dot (an anchor on the cliff or a joint you built), then tap another spot to add a member. Members can be up to 3 m long. Road is what the car drives on - it must connect the two cliffs. Beams make the triangles that hold the road up. Press Test to send the car across. Colours show the force in each member: blue = tension (pulled), red = compression (pushed). Too much force and it snaps!
The physics: every joint is a mass, every member is a stiff spring. Members break above their strength - and compression members are weaker, because long thin struts buckle. Triangles are strong because a triangle can't change shape without stretching a member. That's why real bridges (and cranes, and roofs) are full of them.