Imagine yourself standing in an open flat park. In front of you stands two thin poles, mounted perpendicular to the ground. The tops of the poles are both fifty metres from the ground and are connected by a single piece of inelastic rope that is eights metres long. The lowest part of the rope dangles ten metres above the ground.
The question is; how far apart are the two poles?
The two poles are adjacent to each other, or rather zero metres apart. If you were to take the eight metre long rope, hold both ends, it would now be forty metres long. If each pole is fifty metres tall and touching each other, the bottom of the rope would be ten metres from the ground.
When I was asked this riddle I worked it out by calculating the upper and lower boundaries of the solution. At most, the poles would be eighty metres away, with a taught rope fifty metres from the ground. At least, the poles would be zero metres away, with a completely slack rope, now half it's length, so ten metres off the ground. Ah ha!