Dip one end of the carton into the bubble solution, and gently blow into the other end. They try to pull themselves into the shape that has the smallest surface area, which is a sphere. Blow a bubble and watch the top of the bubble closely. Give another demonstration. Blow several bubbles, and tell your friends which bubble will pop first.
Less light is being reflected, so a black band forms on top of the bubble right before it pops.
Our many thanks to the Soap and Detergent Association for their Bubble Info and Graphics.