The problem is to find a walk through the city that crosses each bridge once and only once. Turns out that this is impossible: I told Sam, but he kept trying anyhow. He ended up pretty frustrated. Later, over supper, I told him that Königsberg was now a Russian city called Kaliningrad – that it had been captured in WWII, in a vast assault involving thousands of tanks (3rd Belorussian Front). Sam, assuming the Soviet motives were the same as his, pointed out that they only really needed to have blown up one bridge.