The beauty of water is that it can be used to express all sorts of emotion, from the most calm and tranquil to the most tumultuous and passionate.
This painting, which depicts the play of sunlight on the water and the waterlilies, expresses something deeper. The colors are dark and moody, this is not a light and playful scene, the peaceful and calm exterior betrays darker depths: pensivity is the facade, what is underneath?
To learn more about Claude Monet, head over to Wikipedia, there is no better place to begin.
