I can’t remember where I saw this but I saw it as a bar trick first. My year 8 class have been studying circles and so I showed them this starter. I got a pint glass and asked the question “Which is bigger the circumference at the top of the glass or the height of the glass?”. It is an optical illusion and most of the class voted for the height. I then put the glass on a thick book and asked again. Virtually all now agreed it had to be the height (with book). However, a bit of string showed it was still the circumference. Try it and see, you might be surprised. Filling the glass with your favourite beverage whilst testing the idea is, of course, optional.