In physics, the “observer effect” describes how the act of measuring something subtly alters that something. Werner Heisenberg and colleagues found this (also called the “uncertainty principle”) as they measured atomic particles, finding their method precisely read a particle's position, but interfered with its motion — making the two dimensions (position and motion) impossible to accurately measure simultaneously. Or as one Wikipedia author noted, checking the pressure in a tire causes air to escape, thereby changing the amount of pressure.