Occasionally there are physics lab demonstrations that I think are pretty awesome but that my students just think are "meh." This is one of those cases. The basic idea in this demo, which I used in my class at Southeastern Louisiana University, is to swing a mass around in a horizontal circle. But wait! There's a cooler part: By running the string through a vertical tube and attaching it to another mass, we can control the tension in the string. It's loads of fun.