This question, more or less, came up on a different site where we also posted the Up, Up, and Away video. What the poster wanted to know was why we didn’t also weight all the strings in addition to the paper clips. If we were trying to find the true buoyant force of helium gas and how much the gas could lift, then, yes, we would need to consider the weight of the strings plus the weight of the balloons. But, our question wasn’t so much about how much helium but rather how many helium balloons it would take to lift a pound. As such, we considered the helium, the balloon, and the string to all be part of the lifting system. Admittedly, it isn’t the most accurate method, but it is what we could afford . Thanks for the great questions!