The centre of mass
The one point a shape behaves as though all its weight is at.
Every shape has a point where its weight can be taken to act. Hold it there and it does not turn; support it anywhere else and it does. For a uniform block that point is the middle; for an L it is out in the air where there is no shape at all, which is why an L balanced on its corner falls over.
A stack stands up as long as the centre of mass of everything above a joint stays over the base underneath it. That is the whole of why a tower of blocks falls when one is nudged: not because it was pushed hard, but because the weight above moved past the edge.
In the app a shape's balance point can be moved off its middle, and the shape then hangs off that point. A meter on a body reads its position, which is the position of the centre of mass rather than of the drawing.
Open the stack scene in the app to watch this one run.