You stand on the shore of a small river that measure 10 meters across. You take your cat and throw it into the middle of the stream. You know your cat can swim very well with 0.2 m/s and that it will come directly back to you. But the river is flowing to the right with 0.1 m/s, so the cat has to compensate for that, to swim to you in a straight line.
How long did it take the cat to reach you?
If you would try this experiment a second time, the cat probably would decide not to come back to you, but instead directly swim to the other shore. How long would the cat need at least to reach the other shore?