Heisenberg’s uncertainty principle
Fundamental law of quantum theory: All physical quantities that can be measured come in pairs. If one of the quantities in a pair is measured with high precision, the corresponding other quantity is necessarily determined only very vaguely. It is impossible to measure precisely and simultaneously both quantities in one and the same pair.
An example for such a pair are the location and the velocity of a quantum particle: Very precise measurements of the location disturb the velocity; if the velocity is measured precisely, it is automatically unclear where exactly the particle is located.