When I was younger and doing bodywork on cars I spent a lot of time squatting flat footed while welding, sanding, etc. My father, who could also do this, pointed out to me that most people cannot. I took advantage of that when hanging out with friends. Whenever physical challenges came up like arm wrestling, cartwheels, handstands, etc, I would offer the flat footed squat challenge.

I would demonstrate and make it look really easy because it was for me. Of course, everyone thought they could do it too, but almost all them fell over backwards as soon as they tried. After reading this I had to check, I can still do it.