Draw a straight line, as shown in the diagram,
from the milking-stool perpendicular to the near bank of the river, and
continue it to the point A, which is the same distance from that bank
as the stool.
If you now draw the straight line from A to the door of the dairy, it
will cut the river at B.
Then the shortest route will be from the stool to B and thence to the
Obviously the shortest distance from A to the door is the straight
line, and as the distance from the stool to any point of the river is
the same as from A to that point, the correctness of the solution will
probably appeal to every reader without any acquaintance with geometry.