The river flows from outskirt of the mountain downward following a very crooked path. The lava from the volcanic activities chooses a similar route. Similarly, a glacier pathway incudes a number of curvatures. Furthermore, The one who is skiing tries to control himself against gravity by following a zigzag path. I conclude that nature is quite nonlinear. What is your opinion? Can we simulate this crooked trajectory by a set of differential equations? I do not think it would be so easy!
Moreover, the engineers use the curvatures to build the mountainous roads and even looking at the railroad tracks, most of them do not follow a straight line.