Walter Lewin's best performance was the pendulum demonstration, and I copy the transcript now:
Would the period come out to be the same or not?
Some of you think it's the same.
Have you thought about it, that I'm a little bit taller than this object and that therefore maybe effectively the length of the string has become a little less if I sit up like this? And if the length of the string is a little less, the period would be a little shorter.
Why would the length decrease? Shouldn't it increase, as the string is stretchable?
If this concerns centre of mass, a diagram would be much appreciated!