OK, here's an easy and nice one:
Points $A$ and $B$ are separated by two rivers. One bridge is to be built across each river so as to minimize the length of the shortest path from $A$ to $B$. (Assume you can't travel on water.) Where should they be placed? Author: Titu Andreescu and Răzvan Gelca
- Each river is an infinite rectangular strip (i.e., each of them has the same width everywhere), and they may not be parallel.
- Each bridge must be a straight segment perpendicular to the sides of the river.
- Assume that $A$ and $B$ are "sufficiently far" from the intersection of the two rivers.
- This is a purely geometrical puzzle; no tricks/ cheating/ wordplay.
- An answer should ideally contain a method for determining the positions of the bridges, and a proof that this method works.