Mini M&Ms on a flat surface form a flag as they dissolve.
To repeat at home: arrange mini M&Ms on a small flat rectangular surface with raised edges. (This one is a lid from a plastic resealable container.) Using an eyedropper, gently drip water into the middle of the rectangle and let it spread out. Make sure that you are not squirting the water in one direction or another--if the water is flowing in any direction, it will push the dissolving color that direction. Above all, do not stir!
Because the candies are regularly spaced, the dissolving solutions push against each other as they spread, creating the stripes.