0. Classic: If a bear walks one mile south, turns left and walks one mile to the east and then turns left again and walks one mile north and arrives at its original position, what is the color of the bear.
北极显然是一个很容易被想到的点，另外的点在南极附近。以南极为圆心，周长为一英里的圆A。再往北增加半径一英里，在这个圆B上起点O向南走一英里到达圆A某点C。再向东一英里绕圆A一周回到点C。再向北走一英里回到起点O。
