Uncertainty principle
We know from everyday life that trying to weigh something on a bathroom scale placed on a washing machine in spin cycle will produce inaccurate results because the needle on the scale will jiggle badly. We can turn off the washing machine. But for very accurate measurements we find that trucks going by in the neighborhood make the needle jiggle, so we can put the scale on a something to insulate it from the outside disturbances. We believe that we can eliminate vibrations enough to give us results just as accurate as we want. We never consider that the thing on the scale is itself vibrating or that it possesses an indefinite momentum.