The bridge job is due Friday and you have a gap between two trestle tables, a bag of iceblock sticks, a pin gun and a budget. Drag between the anchor pins to lay a stick, watch the cost come off, and when you think it will hold, send the tour bus across. The sticks are real bodies and the joints carry real load: a bridge that is beautiful and unbraced snaps in the middle exactly like a real one would, and the bus goes in the ditch with everyone in it. Sticks glow red as they take strain, so a bridge that fails tells you exactly where it was wrong. Three tests a span, which is enough to be wrong twice and not enough to guess.