After posing the initial question: Which string will break when the bottom string is pulled, I have them work through some leading questions.
Instead of having students predict which string will break, I have them request a string for me to break. Once they understand this paradigm shift, they request the bottom string. And I oblige. Eventually I break the top string, too. This can be navigated in the preso, alone. But I prefer to do the demo in my empty classroom because I can.
The efficacy of this demo lies in the dependence of the outcome on the presenter's technique. If it were a 50-50 coin flip each time, the demo would not have any pedagogical value. Some ponderables are offered post-demo, too.
As is a continuing theme in my RT;DLs, the student sheet is a Google Doc and the preso is an Apple Keynote preso, exported to HTML. These exports work delightfully on computers. Not so well on tablets or phones.
This one takes a bit of practice to navigate.
Things shown in images but not in words: strings break when they are stretched beyond their limit by tension greater than the sting can withstand. A rapid pull stretches the bottom string through its limit before the dumbbell moves very much, thus keeping the top string from being stretched. A slow pull allows the top string to be at greater tension than the bottom string, allowing it to reach its limit before the bottom string does.
Student sheet: Demo - The Clever Dumbbell
HTML Preso: Demo - The Clever Dumbbell