Another characteristic of elementary particles is that we can never simultaneously know both their position and speed.

Take a marathon runner heading towards the finish line. It is possible for us to observe her along the way, and say with certainty that she is running at X km per hour at point that is Y km from the start, but we can't say the same for elementary particles.

Imagine a quantum in motion. To investigate its position, we shine light on it, but the energy contained in that light affects the quantum itself, changing its velocity. When a quantum moves from point A to point B, we can calculate speed from the time it took and the distance involved, but only after the event. It is impossible for us to track a quantum in motion and measure its velocity and momentum with equally high precision...

In the ordinary world, we can use sound, light or various other yardsticks to measure the movement or properties of objects. Because of the way that measurement affects the object being measured in the quantum world, however, measuring various aspects of elementary particles is extremely difficult. It is even more difficult than, for example, trying to measure the length of something using a yardstick that stretches and contracts and never maintains a constant length. In this sense, elementary particles are strange entities.

This lack of precision, or the fact that "it is impossible to simultaneously measure and confirm the position and velocity of an elementary particle at any single point in time (one measurement will always be imprecise), was first pointed out in the Heisenberg Uncertainty Principle.