Hinge

your knee is a door. your hip is a joystick.

pose a wooden mannequin until each joint refuses what it cannot do

An artist's mannequin is a stack of joints, each with its own motion. Elbow and knee are the same hinge pointed opposite ways: one folds toward you, one away, neither backward. Pushing each finds hinge, ball and pivot.

Concepts: anatomy, rotation.

An interactive you play with one thumb, on Wundr.