Place a few drops of food colouring in different places in the bowl.
Dip a cotton swab in dish soap.
Dip that cotton swab in the middle of the food colouring.
Explain that a little competition can be beneficial and result in a nice design, but if you do too much it won’t work anymore. More than that, the design becomes ugly (if you use different colours and the blending makes the design disappear).