Events Cannot Occur at the Same Time
When pulling a gumball from the bowl, you will be pulling only one of four colors. You could not pull a gumball and it be a combination of two colors. Since there are only gumballs in the bowl, pulling a gumball and red color would not be mutually-exclusive. We could add marbles to the bowl that are the same four colors as the gumballs. Then, pulling a red color could be mutually-exclusive because it could be either a marble or a gumball.