When you throw a stick in the water and the waves don’t form circles.
Throwing something in the water usually results in waves traveling out in circles from the point of impact. But if you throw your stick into a current, the waves get distorted. Watch the movie below!
Slightly confusing that the stick drifts away, too, so that it doesn’t mark the center of the circle. But still it is clear that waves travel a lot faster downstream than upstream – at least relative to the whole system, not the water ;-)