Prop a piece of white cardboard upright next to the container so that it is situated between the container and the window. Hold a mirror in the water so that it is facing the white cardboard. The curved water surface acts as a prism that separates the colors of sunlight. Adjust the angle of the mirror to shine the rainbow on the cardboard. Observe the colors of the rainbow.
Sunlight is a combination of different colors of light.
These colors always occur in this order with red always showing up on the outside. You can remember these colors by turning them into a name.