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Absolutely precise measurements are impossible, due to interference to the measured quantity which is inevitably introduced by the measuring instrument. The example used by physicists is the impossibility of observing a sub-atomic particle. Electron microscopes shower their subjects with electrons, and translate the electron 'echoes' into images. Electrons, however, are comparable in mass to their subjects and consequently knock them out of their natural states.
A macroscopic example of this phenomenon is the thermometer which measures the temperature of a cup of tea. In the process of measuring, the thermometer touches the tea and lowers the temperature in the cup ever so slightly.