Here’s a fun project you can do in your own kitchen with basic kitchen supplies. This project is all about surface tension. Soap will change the surface tension of the milk but you wouldn’t otherwise notice the change without the dye you will add to the milk. Add some spots of dye and watch it swirl and swirl.
Check out the boys’ instructions and I would use Dawn Dish Soap. I don’t know what there is about Dawn but it’s used in various craft and science projects so I actually keep a bottle on hand. I had otherwise never used it before on actual dishes.