Three masses sit on a plane. Move them, or change how heavy they are, and the gold cross tracks the mass-weighted average of their positions — which is very often a spot where nothing actually is.
Center of mass xcm = Sum(m·x) ÷ Sum(m)
xcm = 4.10 m
ycm = 2.65 m
total mass M = 10.0 kg
The cross is sitting in empty space between the discs.
Mass 1
Mass5.0 kg
Position x2.0 m
Position y1.5 m
Mass 2
Mass2.0 kg
Position x8.0 m
Position y2.0 m
Mass 3
Mass3.0 kg
Position x5.0 m
Position y5.0 m
Plane spans x: 0 to 10 m and y: 0 to 6 m. Each mass is at least 0.1 kg, so the total is never zero and the division is always safe.
Try it: set all three masses equal and the cross lands on the centroid of the triangle. Then push Mass 1 up to 20 kg and watch the cross fall onto it.