Try teaching 80 year olds with hip and knee replacements how to squat (bodyweight, holding onto rails and minimal ROM compared to a normal squat)... can be funny sometimes. They tend to like to bend at the knees and do some weird hip movement forwards. Then you say sit down like you would on the toilet, pushing your hips back. They get it and then about 2 reps later they are back to doing the same thing again.