Thanks for finding this video link, Dr. Nebel! My kids and I tried it and it worked really well to demonstrate the concept! We had to rig up a way to help the cup balance on the battery, but it was a simple demonstration that used household items we already had on hand (water, baking soda, thumb tacks, plastic cup, 9v battery). We happened to have test tubes from another chemistry set but you could still benefit from the demonstration even without the test tubes. You’d still see the bubbles rising, one column having more bubbles than the other. It just wouldn’t be as clear that it’s a 2:1 ratio of hydrogen to oxygen. All that to say, don’t let a lack of test tubes stop you from trying this demonstration.