I was demonstrating a lab once where the students had to use a dual channel scope to measure two different points in the circuit and understand the effect of the grounded scope probe clips - the point being to teach tham that they could make any point arbitrarily ground.
One guy really didn't get it so we measured the resistance between the scope clip and the earth pin on the plug to explain it to him. So he then got 2 single channel scopes and plugged them into different outlets. I think his next plan was to run one of them on an extention cord to another planet.
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